ORGANS OF MOTION. — Swimming is the principal mode of progression of the Cetaceans, but the Phytophagous species appear to have the power, in order to feed upon marine plants, of crawling and walking at the bottom of the sea by means of their anterior members, which in other Cetaceans are exclusively natatory organs.
The head, in all, has so little mobility, that its axis can be but slightly altered, without that of the body altering also.
In the form and composition of the skull the Cetaceans of both tribes present many im portant differences, as compared wtth other rnammiferous animals. In the Herbivorous genera the bones are dense and massive, and where they are not anchylosed their connection is of a loose kind. In the Dugong the skull is more especially remarkable for the large size of the intermaxillary bones ( a, a, figs. 246, 247), which extend backwards as far as the middle of the temporal fossw, and are bent down ante riorly over the symphysis of the lower jaw, so as to terminate nearly on a level with its infe rior margin. This extent and shape is required in the Dugong for the lodgement of the perma nent incisors (b,b), which are developed to a large size, one in each intermaxillary bone, and consequently the nostrils are placed much higher and further from the mouth than in the Manatee, in which, in consequence of the small deciduous incisors having no successors, the intermaxillary bones are of much smaller size. The form of the bony aperture of the nostrils (c, fig. 247) in both the Dugong and Manatee is a large oval, which in the Dugong, as in the typical Cetaceans, is directed upwards. The entire cranium, and especially the frontal hones ( 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, figs.
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,fig. 246), is of larger relative size than in the 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,fig.247) are developed in the fcetus, 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 cristw 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.