The skull of the true or Zoophagous Ceta ceans is characterized by the great breadth and elevation of the cranium, by the almost verti cal direction of the nasal passages, by the de pressed position of the orbits as compared with the bony nostrils,—a character which is still more marked in these than in the herbivorous species; and, lastly, by the extreme prolonga tion of the oral or labial portions of the inter maxillary and maxillary bones. The superior maxillaries (g, g, fig. 268) are also developed posteriorly so as to rise anterior to the frontal bones, over which they are expanded, extending as far as the level of the nasal bones, which form almost the summit of the cranium. Such at least is the general configuration of the skull in the Delphinidee, which constitute the largest family of the Zoophagous tribe.
In the Phocana globiceps, of which the skull is represented in fiR-. 248, the cranium is very convex behind ; the occipital crest (a, a) sur rounds the upper part and descends on each side to the middle of the temporal cristm : the posterior convexity is not formed by the occi pital bone alone, but also by the interparietal and parietal bones (b, b), the whole being an chylosed together at a very early period. The parietal bones descend, as in the human sub ject, between the temporal and the frontal (c, c), and reach the lateral ala of the posterior sphe noid. As the parietals terminate behind the transverse superior cranial or occipital ridge, and the superior maxillary bones approach very close to the same part, the frontal bone seems to be represented by a very narrow osseous band traversing the cranium from right to left, and dilating at each extremity to form the roof of the orbit (d, d). But when the maxillary bones which have extended over the whole anterior part of the cranium are raised, the frontal bone is then seen to be of much larger size than the external appearances indicate.
The two nasal bones (d, d) are in the form of oblong rounded tubercles, set deeply in two depressions in the middle of the frontal bone, and in front of which the nasal passages (e, e) are continued vertically downwards. The two intermaxillaries (f,j) form the exter nal and anterior margin of the nasal apertures. The cribriform plate of the ethmoid consti tutes the posterior wall of the nasal passages; and in this plate there are three or four small perforations. The remainder of the circum ference of the bony nostrils is formed by the maxillary bones, of which a small part appears at g : their septum is the vomer, which is joined to the ethmoid as usual.
The malar bone is an irregular flattened bone, which assists the frontal in forming the orbit, and, like it, is covered by the maxillary bone : it sends backwards a long and slender process, which articulates with the zygomatic process of the temporal bone, and forms the only bony boundary of the lower part of the orbit. The zygomatic process of the temporal bone is united to the post-orbital process of the frontal, bounding the orbit posteriorly; and thus the zygomatic arch is exclusively formed by the temporal bone : this bone terminates at the temporal ridge, having but a small extent of development on the side of the cranium, and not entering at all into the composition of the posterior convex surface. At the base of the cranium the basilar and the lateral occipitals develop expanded plates, which join the ptery goideal of the sphenoid, and a lamina of the temporal bone to which the petrous and tympanic bones have a ligamentous attach ment. The parietal bones also extend behind the temporals, to aid in completing the basilar walls of the cranial cavity, so that the temporal bone is almost excluded from entering into the composition of the cranium, serving merely to close some small vacancies left by the parietals : this structure is of great interest, as we perceive in it the commencement of that displacement of the temporal bones from the cranial parietes which is characteristic of the small-brained and cold-blooded classes of Vertebrata.
The differences between the Dugong and Manatee in respect to the structure of the cranium, we have seen to resolve themselves almost entirely into the expansion and elonga tion of the intermaxillary bones in relation to the tusks, which they are destined to support in the former animal; and we shall find on a com parison of the skulls of the Delphinidec toge ther, that they also differ from one another, chiefly in the forms and proportions of their maxillary and intermaxillary bones.
The Delphinorhynchi are characterized, first, by an extremely narrow rostrum, the length of which is four times greater than that of the cranium ; secondly, by the anterior curvature of the posterior extremities of the intermaxil laries, which, as it were, draw forwards in the same direction the maxillary, the frontal, and even the occipital bones ; thirdly, by the posi tion of the na.sal bones, which are stink in between the frontals and intermaxillaries ; fourthly, by the very diminutive size of the temporal fossw.