Classification of Skeletal Elements

Bones may be grouped according to shape, structure, function, origin, or position. Heterotopic bones are defined by position and may be located anywhere in the body. The os penis or baculum is an example of such a bone. It is located in the glans of the penis and can be found in all mammals except humans, whales, and some others (Chaine 1926). It functions to stiffen the glans and dilate the fundus of the vagina. Its homolog in the female is the os clitoris, which is more restricted in occurrence and usually absent in the dog (see Chapter 9, Urogenital System). The total average number of bones in each division of the skeletal system, as found in an adult dog (Figs. 4-1 and 4-2), is given in Table 4-1. (Sesamoid bones associated with the limbs are included.) In this enumeration, the bones of the dewclaw (the first digit of the hindpaw) are not included, because this digit is absent in many breeds of dogs, and in other breeds a single or double first digit is required for show purposes (American Kennel Club, 2006). Because dewclaws are nonfunctional, but are frequently injured or ingrown and require treatment, they are often removed.

Classification of Bones According to Shape

Bones may be classified in various ways. Anatomists have long grouped bones according to shape, although borderline forms exist. For descriptive purposes five general divisions on this basis are recognized: long bones, short bones, sesamoid bones, flat bones, and irregular bones. Long, short, and sesamoid bones are found in the limbs, whereas the flat and irregular bones are characteristic of the skull and vertebral column. The terms are readily understandable, except possibly sesamoid, which is derived from the Greek word for a seed that is small, flat, and ovate. Sesamoid bones vary from tiny spheres to the slightly bent, ovoid patella that is 2 cm or longer in a large dog. Some sesamoid elements never ossify but remain as cartilages throughout life, such as those of the distal interphalangeal joints.

Long bones (ossa longa) are characteristic of the limbs. The bones of the thigh and arm, that is, the femur and humerus, are good examples. Typically a long bone, during its growth, possesses a shaft, or diaphysis, and two ends, the epiphyses (Haines 1942). During development each end is separated from the shaft by a plate of growing cartilage, the physeal cartilage.

The epiphysial cartilage (cartilago epiphysialis) is the cartilage on the articular surface of the epiphysis. The rapidly growing, flared end of the bone between the shaft and the epiphysis is called the metaphysis. At maturity the physeal cartilage ceases to grow, and the epiphysis fuses with the shaft as both share in the bony replacement of the physeal cartilage. Farnum and Wilsman (1989) and Farnum et al. (1990) have studied chondrocytes of the growth plate cartilage in situ using differential interference contrast microscopy and time-lapse cinematography. They were able to visualize living hypertrophic chondrocytes as they pass through a sequence of phases, including proliferation, hypertrophy, and death at the chondroosseous junction. Fractures sometimes occur at the physis. Usually after maturity no distinguishable division exists between epiphysis and diaphysis. The ends of most long bones enter into the formation of freely movable joints. Long bones form levers and possess great tensile strength. They are capable of resisting many times the stress to which they are normally subjected. The stress on long bones is both through their long axes, as in standing, and at angles to these axes, as exemplified by the pull of muscles that attach to them. Although bones appear to be rigid and not easily influenced by the soft tissues that surround them, soft tissues actually do contour the bones. Indentations in the form of grooves are produced by blood vessels, nerves, tendons, and ligaments that lie adjacent to them, whereas roughened elevations or depressions are produced by the attachments of tendons and ligaments. The ends of all long bones are enlarged and smooth. In life, these smooth surfaces are covered by a layer of hyaline cartilage, as they enter into the formation of joints. The enlargement of each extremity of a long bone serves a dual purpose. It diminishes the risk of dislocation and provides a large bearing surface for the articulation. The distal end of the terminal phalanx of each digit is an exception to the stated rule. Because it is covered by horn and is not articular, it is neither enlarged nor smooth.

Short bones (ossa brevis) are confined to the carpal and tarsal regions, which contain seven bones each. They vary in shape from the typical cuboidal shape with six surfaces to irregularly compressed rods with only one flat, articular surface. In those bones having many surfaces, at least one surface is nonarticular. This surface provides an area where ligaments may attach and blood vessels may enter and leave the bone.

Sesamoid bones (ossa sesamoidea) are present near freely moving joints. They are usually formed in tendons, but they may be developed in the ligamentous tissue over which tendons pass. They usually possess only one articular surface, which glides on a flat or convex surface of one or more of the long bones of the extremities. Their chief function is to protect tendons at the places where greatest friction is developed.

Flat bones (ossa plana) are found in the limb girdles, where they serve for muscle attachment, and in the head, where they surround and protect the sense organs and brain as well as serve for muscle attachment. The bones of the face are flat, providing maximum shielding without undue weight, and streamlining the head. Furthermore, the heads of all quadrupeds overhang their centers of gravity; a heavy head would be a handicap in locomotion. The flat bones of the cranium consist of outer and inner tables of compact bone and an intermediate uniting spongy bone, called diploë. In certain bones of the head the diploë is progressively invaded during growth by extensions from the nasal cavity that displace the diploë and cause a greater separation of the tables than would otherwise occur. The intraosseous air spaces of the skull formed in this way are known as the paranasal sinuses. Bones that contain air cavities are called pneumatic bones (ossa pneumatica).

Irregular bones (ossa irregulata) are those of the vertebral column, but the term also includes all bones of the skull not of the flat type, and the three parts of the hip bone (os coxae). Jutting processes are the characteristic features of irregular bones. Most of these processes are for muscular and ligamentous attachments; some are for articulation. The vertebrae of quadrupeds protect the spinal cord and furnish a relatively incompressible bony column through which the propelling force generated by the pelvic limbs is transmitted to the trunk. The vertebrae also partly support and protect the abdominal and thoracic viscera, and give rigidity and shape to the body in general. The amount of movement between any two vertebrae is small, but the combined movement permitted in all the intervertebral articulations is sufficient to allow considerable mobility of the whole body in any direction (Slijper, 1946).

Development of Bone

Bone consists of cells in a specialized intercellular organic matrix called osteoid, which is mineralized primarily by hydroxyapatite. The cells that direct the formation of cartilage and bone may be derived either from mesoderm or from neural crest (Hall, 1988; Noden & de Lahunta, 1985). The most abundant protein of the organic matrix of bone is type I collagen, which gives bone its structural support and strength. However, bone matrix contains numerous other matrix macromolecules, including collagen types III and V, proteoglycans, lipids, morphogenetic proteins, and enzymes as well as phosphoproteins and glycoproteins specific to bone, such as osteocalcin, osteonectin, and osteopontin. The function of these bone-specific proteins is a major area of current research, and in the future many new noncollagenous proteins of bone will undoubtedly be discovered.

Cartilage, often a precursor of bone, has been reviewed in books by Hall (1977) and Hall and Newman (1991), who considered developmental and molecular aspects of cartilage (see Chapter 2 for a discussion of fetal bone development and illustrations of ossification sequences in various bones).

Bone-forming cells, or osteoblasts, are capable of synthesizing extracellular collagenous and noncollagenous proteins and proteoglycans, the building blocks of bone matrix. They also respond to circulating hormones and produce growth factors that mobilize osteoclast precursor cells. Osteoblasts on the bone surface become osteocytes as they are surrounded by mineralized matrix (Bonewald, 2008). Each bone cell or osteocyte rests in a lacuna and has long branching processes that extend through canaliculi in the mineralized matrix to lacunae of neighboring cells. All bone-lining cells are interconnected and appear capable of maintaining active transport in calcium homeostasis.

The formation of bone by osteoblasts and the resorption of bone by osteoclasts are linked, or “coupled,” in ways that are not completely understood. However, the controlling cell for bone remodeling, which goes on throughout life, is the osteoblast. Mechanical stress via muscle attachment, nutrition, vitamin D, calcitonin, parathormone, and sex hormones plays a great role in bone remodeling throughout life. In old age some bone cells die, whereas some become “uncoupled” and are not replaced, thus disrupting bone metabolism. This results in the thinning of cortical as well as trabecular bone. Resorption or deposition may be excessive and result in clinical problems, some of which still defy treatment. For a consideration of over-nutrition and skeletal disease, see Wu (1973).

For an excellent review of bone cell biology, which the authors contend is still in its infancy, see Marks and Popoff (1988). Recent books on the osteoblast and osteocyte and the osteoclast are part of a seven-volume series titled Bone: A Treatise, edited by B. K. Hall (1989-1992). This is a timely update for the still useful four-volume, second edition of The Biochemistry and Physiology of Bone, by Bourne (1972-1976). In the new series by Hall, there will be 74 chapters by 127 authors.

The fetal skeleton (see Chapter 2) is characterized by bones formed in membrane (intramembranous) that precede or accompany bones formed in cartilage (endochondral). Both intramembranous bone and endochondral bone are remodeled during development and form lamellar bone with haversian systems indistinguishable from each other. The terms membrane bone and cartilage bone refer to the primary tissue being mineralized. Almost all so-called cartilage bones begin their ossification beneath a perichondral membrane, followed by vascular invasion and endochondral ossification. Several membrane bones develop secondary cartilage after membranous ossification has begun. This secondary cartilage ossifies to form compact bone indistinguishable from the remainder of the structure.

For further information, see Calcified Tissue Research, an international journal founded in 1967 and devoted to the structure and function of bone and other mineralized systems, and Developmental and Cellular Skeletal Biology by Hall (1978).

The bones of the face and dorsum of the cranium develop in sheets of connective tissue, not in cartilage. This type of bone formation is known as intramembranous ossification. Osteoblasts and osteoclasts continue to be the laborers in this activity. The compact bone formed by the periosteum is identical with membrane bone in its elaboration. Bony tissue of either type is capable of growing in any direction. The jaws and hyoid arches are preceded by cartilages, which are derived from the neural crest. In the proceedings of the Third International Conference on Bone (Dixon et al., 1991) there are more than 50 papers that consider present methods for studying the growth of cartilage and bone. A common technique for studying developing cartilages and bones in the fetus is the use of color stains and the subsequent clearing of tissues. For cartilage, Alcian blue or toluidine blue is used to stain the mucopolysaccharide and for bone, alizarine red, combined with calcium to stain them red. Subsequent maceration of tissues with sodium or potassium hydroxide followed by clearing in glycerine, benzyl benzoate or ethylene glycol make cartilage and bone formation visible. Examples of such staining can be seen in Chapter 2. (Evans 1948, Orsini 1962, Crary 1962, Dingerkus & Uhler 1977, Kelly & Bryden 1983, Taylor & Van Dyke 1985).

Structure of Bone

The gross structure of a dried, macerated bone is best revealed if the bone is sectioned in various planes. Two types of bone structure are seen. One is compact, or dense, which forms the outer shell of all skeletal parts. The other is spongy, or cancellous, which occupies the interior of the extremities of all long bones and the entire interior of most other bones, except certain of the skull bones and the bones of the thoracic and pelvic girdles. Spongy bone is not found in the girdles, where the two compact plates are fused.

Compact bone (substantia compacta and substantia corticalis) is developed in direct ratio to the stress to which the bone is subjected. It is thicker in the shafts of long bones than in their extremities. It attains its greatest uniform thickness where the circumference of the bone is least. The maximum thickness of the compact bone found in the femur and humerus of an adult Great Dane is 3 mm. Local areas of increased thickness are present at places where there is increased tension from muscles or ligaments.

Spongy bone (substantia spongiosa) is elaborated in the extremities of long bones, forms the internal substance of short and irregular bones, and is interposed between the two compact layers of most flat bones. Spongy bone consists of a complicated maze of crossing and connecting osseous leaves and spicules that vary in shape and direction. The spongy bone of the skull is known as diploë.

The shafts of long bones in the adult are filled largely with yellow bone marrow (medulla ossium flava). This substance is chiefly fat. In the fetus and the newborn, red bone marrow (medulla ossium rubra) occupies this cavity and functions in forming red blood cells. No spongy bone is present in the middle of the shaft of a long bone, and the marrow-filled space thus formed is known as a medullary cavity (cavum medullare).

Spongy bone is developed where greatest stress occurs. The leaves or lamellae and bars are arranged in planes where pressure and tension are greatest, this structural development for functional purposes being best seen in the proximal end of the femur. The interstices between the leaves and the bars of spongy bone are occupied by red marrow. The spongy bone of ribs and vertebrae and of many other short and flat bones is filled with red marrow throughout life. In the emaciated or the extremely aged, red marrow gives way to fatty infiltration.

The periosteum is an investing layer of connective tissue that covers the nonarticular surfaces of all bones in the fresh state. The connective tissue covering of cartilage, known as perichondrium, does not differ histologically from periosteum. Perichondrium covers only the articular margins of articular cartilages but invests cartilages in all other locations. Periosteum blends imperceptibly with tendons and ligaments at their attachments. Muscles do not actually have the fleshy attachment to bone that they are said to have, since a certain amount of connective tissue, periosteum, intervenes between the two. At places where there are not tendinous or ligamentous attachments it is not difficult, when bone is in the fresh state, to scrape away the periosteum.

The endosteum is similar in structure to periosteum but is thinner. It lines the large medullary cavities, being the condensed peripheral layer of the bone marrow. Both periosteum and endosteum, under emergency conditions, such as occur in fracture of bone, provide cells (osteoblasts) that aid in repair of the injury. Sometimes the fractured part is over-repaired with bone of poor quality. Such osseous bulges at the site of injury are known as exostoses.

Mucoperiosteum is the name given to the covering of bones that participate in forming boundaries of the respiratory or digestive system. It lines all of the paranasal sinuses and contains mucous cells.

Physical Properties of Bone

Bone is about one-third organic and two-thirds inorganic material. The inorganic matrix of bone has a microcrystalline structure composed principally of calcium phosphate. The exact constitution of the crystal lattice is still under study, but it is generally agreed that bone mineral is largely a hydroxyapatite with adsorbed carbonate. Some consider that it may exist as tricalcium phosphate hydrate with adsorbed calcium carbonate (Dixon & Perkins, 1956). The organic framework of bone can be preserved while the inorganic part is dissolved. A 20% aqueous solution of hydrochloric acid will decalcify any of the long bones of a dog in approximately 1 day. Such bones retain their shape but are pliable. A slender bone, such as the fibula, can be tied into a knot after decalcification. The organic material is essentially connective tissue, which on boiling yields gelatin.

Surface Contour of Bone

Much can be learned about the role in life of a specific bone by studying its eminences and depressions. There is a functional, an embryologic, or a pathologic reason for the existence of almost every irregularity. For age-related changes, see Simon et al. (1988).

Most eminences serve for muscular and ligamentous attachments. Grooves and fossae, in some instances, serve a similar function. Facets are small articular surfaces that may be flat, concave, or convex. Trochleas and condyles are usually large articular features of bone. The roughened, enlarged parts that lie proximal to the condyles on the humerus and femur are known as epicondyles.

Vessels and Nerves of Bone

Bone, unlike cartilage, has both a nerve and a blood supply. Long bones and many flat and irregular bones have a conspicuous nutrient (medullary) artery and vein passing through the compact substance to serve the marrow within. Such arteries pass through a nutrient foramen (foramen nutricium) and canal (canalis nutricius) of a bone and, on reaching the marrow cavity, divide into proximal and distal branches that repeatedly subdivide and supply the bone marrow and the adjacent cortical bone. In the long and short bones, terminal branches reach the physeal plate of cartilage, where, in young animals, they end in capillaries. In adults it is likely that many twigs nearest the epiphyses anastomose with twigs arising from vessels in the periosteum. Nutrient veins pursue the reverse course. Not all of the blood supplied by the nutrient artery is returned by the nutrient vein or veins; much of it, after traversing the capillary bed, returns through veins that perforate the compact bone adjacent to the articular surfaces at the extremities of these bones. The periosteal arteries and veins are numerous but small; these arteries supply the extremities of long bones and much of the compact bone also. They enter minute canals that lead in from the surface, and ramify proximally and distally in the microscopic tubes that tunnel the compact and spongy bone. The arterioles of the nutrient artery anastomose with those of the periosteal arteries deep within the compact bone. It is chiefly through enlargement of the periosteal arteries and veins that an increased blood supply and increased drainage are obtained at the site of a fracture. Veins within bone are devoid of valves, the capillaries are large, and the endothelium from the arterial to the venous side is continuous. Lymph vessels are present in the periosteum as perivascular sheaths and probably also as unaccompanied vessels within the bone marrow.

The nerves in bone are principally sensory and evidence has been accumulating that the nervous system plays a crucial role in remodeling of bone and the maintenance of bone mass (Martin & Sims, 2009). It is thought that approximately 10% of the human skeleton is replaced each year by remodeling. Both the central and sympathetic part of the peripheral nervous systems are believed to be involved in such regulation. Neuroendocrine controls include leptin, which inhibits bone formation. Sensory nerves carry impulses that may result in pain. Kuntz and Richins (1945) state that both the afferent and the sympathetic efferent fibers probably play a role in reflex vasomotor responses in the bone marrow.

Function of Bone

The skeleton of the vertebrate body serves four functions.

Occipital Bone

The occipital bone (Figs. 4-5, 4-10, 4-11, 4-44 and 4-48) forms a ring, the foramen magnum, around the junction of the medulla oblongata and the spinal cord. The ring develops from four centers: a squamous part dorsally, two lateral condylar parts, and a basilar part ventrally. A keyhole-shaped notch may be present dorsally (Fig. 4-12). This normal feature is common in the brachycephalic toy breeds (Watson, 1981).

The squamous part (squama occipitalis), also known as the supraoccipital, is the largest division. This bone forms the dorsal border of the foramen magnum and hides the cerebellum. An unpaired, median, interparietal bone (Fig. 2-39) makes its appearance on about the forty-fifth day of gestation. It is superficial to the paired parietal bones and to the supraoccipital bone. Usually, it fuses with the dorsorostral border of the supraoccipital bone forming part of the saggital crest, but in some dogs, it remains as a separate entity, the os interparietale. Occasionally an unfused interparietal bone is found in an adult dog. It may be more apparent inside the cranium than externally. Erhart (1943) examined 127 dog skulls for the presence of a separate interparietal bone, and he found 17 examples in 33 brachycephalic skulls; 9 in 30 mesaticephalic skulls; and none in 64 dolichocephalic skulls. Of the 14 brachycephalic fetuses studied, 12 had a distinct ossicle, whereas only 1 of 5 dolichocephalic fetuses had an independent ossicle. In the Beagle fetuses studied by Evans (1974) there was always a separate median interparietal bone for a brief period that fused indistinguishably with the squamous part of the supraoccipital bone before birth. From the interparietal process arises the middorsal external sagittal crest (crista sagittalis externa), which, in some specimens, is confined to this bone. The rostral end of the interparietal process is narrower and thinner than the caudal part, which turns ventrally to form a part of the caudal surface of the skull. The nuchal crest (crista nuchae) marks the division between the dorsal and the caudal surfaces of the skull. It is an unpaired, sharp-edged crest of bone that reaches its most dorsal point at the external occipital protuberance. On each side it arches ventrally before ending on a small eminence located dorsocaudal to the external acoustic meatus. The external occipital protuberance (protuberantia occipitalis externa) is the median, triangular projection forming the most dorsocaudal portion of the skull. The external occipital crest (crista occipitalis externa) is a smooth median ridge extending from the external occipital protuberance to the foramen magnum. It is poorly developed in some specimens.

Within the dorsal part of the occipital bone and opening bilaterally on the cerebral surface is the transverse canal (canalis transversus), which, in life, contains the venous transverse sinus. The transverse canal is continued laterally, on each side, by the sulcus for the transverse sinus (sulcus sinus transversi). Middorsally, or to one side, the dorsal sagittal sinus enters the transverse sinus via the foramen for the dorsal sagittal sinus (foramen sinus sagittalis dorsalis). Between the laterally located sulci the skull protrudes rostroventrally to form the internal occipital protuberance (protuberantia occipitalis internus). Extending rostrally from the internal occipital protuberance is the variably developed, usually paramedian, and always small internal sagittal crest (crista sagittalis interna). The vermiform impression (impressio vermialis), forming the thinnest part of the caudal wall of the skull, is an irregular excavation of the median portion on the cerebellar surface of the squamous part of the occipital bone that houses a part of the vermis of the cerebellum. The vermiform impression is bounded laterally by the paired internal occipital crest (crista occipitalis interna), which is usually asymmetric and convex laterally. Lateral to the internal occipital crest, as well as on the ventral surface of the interparietal process, there are elevations, juga cerebralia et cerebellaria, and depressions, impressiones digitatae. Ventrally the squamous part is either curved or notched to form the dorsal part of the foramen magnum. On either side the squamous part is fused with the lateral part. This union represents the former articulation (synchondrosis intraoccipitalis squamolateralis), which extended from the foramen magnum to the temporal bone.

The paired lateral parts (partes laterales), also known as exoccipital parts, bear the occipital condyles (condyli occipitales), which are convex and, with the atlas, form the atlantooccipital joints. The paracondylar process (processus paracondylaris) is located, one on either side, lateral to the condyle and ends in a rounded knob ventrally, usually on a level with the ventral portion of the rostrally located tympanic bulla. Between the paracondylar process and the occipital condyle is the ventral condyloid fossa (fossa condylaris ventralis). On a ridge of bone rostral to this fossa is the hypoglossal canal (canalis n. hypoglossi), which is a direct passage through the ventral part of the occipital bone for the hypoglossal nerve. The dorsal condyloid fossa (fossa condylaris dorsalis) is located dorsal to the occipital condyle. The rather large condyloid canal (canalis condylaris) that contains the basilar sinus runs through the medial part of the lateral part of the occipital bone. There is an intraosseous passage between the condyloid canal and the hypoglossal canal. Usually there is also a small passage between the condyloid canal and the petrobasilar fissure.

The basilar part (pars basilaris), also referred to as the basioccipital part, is unpaired and forms the caudal third of the cranial base. The central dorsal surface of the basioccipital part is concave to form the pontine impression (impressio pontina) rostrally and the impressssion for the medulla oblongata (impressio medullaris) caudally. It is roughly rectangular, although caudally it tapers to a narrow, concave end that forms the central portion of the intercondyloid notch (incisura intercondyloidea). The adjacent occipital condyles on each side deepen the incisure as they contribute to its formation. The incisure bounds the ventral part of the foramen magnum. The foramen magnum is a large, transversely oval opening in the caudoventral portion of the skull, through which the medulla is continuous with the spinal cord and their associated structures: the meninges, vertebral venous sinuses, the spinal portion of the accessory nerve, and the various arteries associated with the spinal cord. In brachycephalic breeds the foramen is more circular than oval, and it is frequently asymmetric or notched. The dorsal boundary of the foramen magnum is featured by the caudally flared ventral part of the squamous part of the occipital bone. The caudal extension is increased by the paired nuchal tubercles (tubercula nuchalia). The lateral surfaces of the caudal half of the basioccipital part fuse with the lateral parts along the former ventral intraoccipital synchondrosis (synchondrosis intraoccipitalis basilateralis). The ventral surface of the basioccipital part adjacent to the petrotympanic synchondrosis possesses muscular tubercles (tubercula muscularia). These are rough, sagittally elongated areas, located medial to the smooth, rounded tympanic bullae. The longus capitis muscles attach here. The pharyngeal tubercle (tuberculum pharyngeum) is a single triangular rough area rostral to the intercondyloid incisure. Laterally the basioccipital bone is grooved to form the sulcus for the ventral petrosal sinus (sulcus sinus petrosi ventralis), which concurs with the pyramid of the temporal bone to form the petrooccipital canal (canalis petrooccipitalis) which contains the ventral petrosal sinus.

Ventrally the rostral end of the basioccipital part articulates with the body of the basisphenoid bone at the cartilaginous sphenooccipital joint (synchondrosis sphenooccipitalis). Ventrolaterally the occipital bone articulates with the tympanic part of the temporal bone to form the cartilaginous occipitotympanic joint (sutura occipitotympanica). Dorsal to this joint is the important petrooccipital suture (sutura petrooccipitalis), in which the jugular foramen opens. The joint between the petrosal part of the temporal bone and the occipital bones that forms the petrooccipital suture is the synchondrosis petrooccipitalis. Laterally, and proceeding dorsally, the occipital bone first articulates with the squamous part of the temporal bone superficially, the occipitosquamous suture (sutura occipitosquamosa), and with the mastoid process of the petrous part of the temporal bone deeply, the occipitomastoid suture (sutura occipitomastoidea); further dorsally it articulates with the parietal bone, the lambdoid suture (sutura lambdoidea). Where the squamous and lateral parts of the occipital bone articulate with each other and with the mastoid process of the temporal bone, the mastoid foramen (foramen mastoideum) is formed. This foramen contains the caudal meningeal vessels.

Variations in the occipital bone are numerous. The foramen magnum varies in shape and is not always bilaterally symmetric (Figs. 4-12 and 4-13) (Simoens et al., 1994; Watson et al., 1989). The condyloid canal may be absent on one or both sides. Even when both canals are present, connections between the hypoglossal and the condyloid canals may fail to develop. The paracondylar processes may extend several millimeters ventral to the tympanic bullae so that they will support a skull without the mandibles when it is placed on a horizontal surface; conversely, they may be short, retaining the embryonic condition. The vermiform impression may be deep, causing a caudomedian rounded, thin protuberance on the caudal surface of the skull. The foramen for the dorsal sagittal sinus may be double. It is rarely median in position. A sutural bone may be present at the rostral end of the interparietal process.