The tympanic is ankylosed to the periotic, but not to the squamosal; it generally develops a tubular meatus, which in the Hare is directed upwards and backwards, in the Beaver, outwards and forwards. In the Porcupines, as well as in most of the smaller Rodents, the meatus is short. In the Capybara it is fissured below.

There is always a considerable tympanic bulla, which is often supplemented by a bulla developed above the tympanic cavity apparently in the periotic. In some genera (Pedetes, Dipus, Chinchilla) this attains an enormous size (see Fig. 59, Per), and forms a rounded prominence on the posterior external angle of the skull, interposed between the squamosal, parietal, and occipital. Usually, the mastoid portion of the periotic only appears on the surface for a small space in front of the exoccipital. In the Beaver, it forms a conspicuous

angular process.

The periotic is never ankylosed with any of the bones of the cranium, other than the tympanic. On its inner surface the floccular fossa is nearly always wide and deep, but it is absent, or nearly so, in the Capybara, Paca, and Porcupine. The place of attachment of the hyoid arch is an inconspicuous depression in the usual situation, and the tympanohyal is never distinct. This is in relation with the rudimentary condition of the anterior cornu of the hyoid.

In the mandible, the symphysial portion is narrow, curving upwards, and rounded laterally, with a single large alveolus on each side. The coronoid process is never large, and is often rudimentary or absent, in relation to the small development of the temporal fossa and muscle; while the portion. adjacent to the angle is greatly developed, showing marked masseteric and pterygoid fossæ, and often with its lower edge expanded laterally, or slightly incurved. The angle is rounded in the Hares, but it is more often produced into a

long backward process, more or less pointed and upturned. The condyle is considerably elevated, with a rounded articular surface, usually longer from before backwards than from side to side.

The hyoid has a transversely extended basihyal and a straight compressed rod-like thyrohyal, often ankylosed with the basihyal. The anterior arch is long, but mostly ligamentous, the ossification being usually confined to the lower part (cerato- and epi-hyals). In the Hares, the basihyal is deep from above downwards, and compressed, keeled, or pointed in front.

CHAPTER XI.

THE SKULL IN THE UNGULATA, HYRACOIDEA, AND
PROBOSCIDEA.

Order UNGULATA; Sub-order Perissodactyla. — In the Horse the whole skull is greatly elongated, chiefly in consequence of the immense size of the face as compared with the hinder or true cranial portion. The basal line of the skull from the lower border of the foramen magnum to the incisor border of the palate is very nearly straight. The occipital and ethmoid planes are nearly perpendicular to this line, the latter inclining slightly forwards. The tentorial

plane, strongly marked by inward projecting ridges of bone, slopes obliquely backwards at an angle of 45°. The cerebral fossa is a smooth and regular oval, broad and rounded in front, and with no distinct division into anterior and posterior portions. The olfactory fossa is short, but deep from above downwards. The pituitary fossa is very shallow, and there are no distinct clinoid processes. The alisphenoid is very obliquely perforated by the foramen rotundum, but the foramen ovale is confluent with the large foramen lacerum medium behind. There are considerable frontal and sphenoidal air sinuses, but the former do not extend any great distance over the brain-cavity.

In front of the cerebral cavity, the great tubular nasal cavities are provided with well-developed turbinal bones, and are roofed over by very large nasals, broad behind, and ending in front in a narrow decurved point. The opening of the anterior nares is prolonged backwards on each side of the face between the nasals and the elongated slender premaxillæ. The latter expand in front, and are curved downwards to form the semicircular alveolar border which supports the large incisor teeth.

The orbit is rather small in proportion to the size of the whole skull, but very distinctly marked, being completely surrounded by a strong ring of bone with prominent edges. The lacrymal occupies a considerable space on the flat surface of the cheek in front of the orbit, and below it the malar does the same. The latter sends a horizontal or slightly ascending process backwards below the orbit, to join the under surface of the zygomatic process of the squamosal, which is remarkably large, and instead of ending as usual, behind the orbit, runs forwards to join the greatly developed postorbital process of the frontal, and even forms part of the posterior and inferior boundary of the orbit-a very exceptional arrangement (see Fig. 60).

The palate is very narrow in the interval between the incisor and molar teeth, in which are situated the large anterior palatine foramina. Between the molar teeth it is broader, but it does not extend further back than the penultimate molar and ends in a rounded excavated border. It is mainly formed by the maxillæ, as the palatines are very narrow. The pterygoids are delicate slender slips of bone attached to the hinder border of the palatines, and supported externally by, and generally ankylosed to, the rough pterygoid plates of the alisphenoid, with no pterygoid fossa between. They slope very obliquely forwards, and end in curved,

compressed, hamular processes. There is a distinct alisphenoid canal for the passage of the internal maxillary artery. The base of the cranium is long and narrow. The glenoid surface for the articulation of the mandible is greatly extended transversely, concave from side to side, convex from before backwards in front and hollow behind, and is bounded

FIG. 60.-Side view of the posterior part of the skull of a Horse, Fr frontal (the line points to the postorbital process); Sq squamosal; Pa parietal; SO supraoccipital; Ex exoccipital: oc occipital condyle; pp paroccipital process; Per mastoid portion of periotic; pt post-tympanic process of squamosal; th tympanohyal; Ty tympanic; pg postglenoid process of squamosal; As alisphenoid (the line points to the plate of the bone which bridges over the alisphenoid canal); Ma malar.

posteriorly, at its inner part, by a prominent postglenoid process (Fig. 60, pg).

The squamosal (Sq) enters considerably into the formation of the temporal fossa, and besides sending the zygo. matic process forwards, it sends down behind the meatus auditorius a post-tympanic process (pt), which aids to hold in place the otherwise loose tympano-periotic bone.