CETACEA. 



565 



Fig. 247. 



Skull of the Dugong. 



(d, d), are consequently proportionally shorter 

 than in the Manatee. The processes of the 

 frontal bone, which form the superior boundary 

 of the orbits, are thinner and more rugose in 

 the Dugong; the portion of the superior max- 

 illary bone, which serves as the floor of the 

 orbit, is narrower ; the malar bone (e, e, Jigs. 

 246, 247), which forms by its curvature the 

 anterior and inferior margins of the orbit, is 

 more compressed and descends lower down. 

 The lachrymal bone, which is situated at the 

 anterior angle of the orbit (f,Jig. 246), is of 

 larger relative size than in trie Manatee ; but, 

 as in that species, it is imperforate. The zygo- 

 matic process of the temporal bone (g,figs. 

 246, 247), which, in the Manatee, is propor- 

 tionally thicker than in any other animal, is of 

 more ordinary dimensions in the Dugong, being 

 more compressed, and extended further back- 

 wards. The connexions of the bones of the 

 cranium are the same in both these herbivorous 

 species. The parietal bones (h, Jig. 247) are 

 developed in the foetus, as usual, each from a 

 distinct centre of ossification ; but, what is 

 very remarkable, the ossification of the inter- 

 parietal bone also proceeds from two lateral 

 and symmetrical points : these four, originally 

 distinct bones, are, however, very early anchy- 

 losed together, and also to the superior occipi- 

 tal bone, which latter junction takes place be- 

 fore the three other elements of the occipital 

 bone have coalesced. The parietal cristae are 

 widely separated from each other. The occiput 

 is narrower, and its crest is less marked than 

 in the Manatee. In the interior of the cranium 

 we may observe that there is no bony tento- 

 rium, and that the cribriform plate of the 

 ethmoid is reduced to two simple depressions, 

 widely separated from one another, and termi- 

 nating anteriorly in two or three small foramina. 

 There is no sella turcica for the pituitary gland. 



The optic foramen presents the form of a long 

 and narrow canal. 



The lower jaw (i,fig. 246) corresponds in 

 depth to the curvature and length of the inter- 

 maxillary bones, and is bent downwards at the 

 symphysis in a corresponding direction, pre- 

 senting on the anterior surface of this part three 

 or four rough and shallow alveoli, in two of 

 which Sir Everard Home* discovered a small 

 rudimental incisor. 



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, Jig. 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 Delphinid*, which constitute the largest 

 family of the Zoophagous tribe. 



In the Phoceena globiceps, of which the skull 

 is represented in Jig. 248, the cranium is very 

 Fig. 248. 



Skull of the Roundfieaded Porpesse ; 

 Phocaena globiceps. 



convex behind ; the occipital crest (a, a) sur- 

 rounds the upper part and descends on each 

 side to the middle of the temporal cristse : 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 

 * See PI. xiv. Philos. Trans. 1820. 



