200 THE SKULL. [chap. 



which may be taken as a type of the order, is remarkable 

 for the massiveness and density of structure of the bones of 

 which it is formed. There are no air sinuses in any part, 

 and most of the bones when cut through appear as hard and 

 solid as ivory. This character is not peculiar to the skull, 

 but shared with it by the ribs, and other bones, and must 

 add much to the general specific gravity of this slow-moving 

 animal, and aid in keeping it to the bottom of the shallow 

 water in which it dwells, while feeding on fuci and other 

 aquatic vegetables. 



The cerebral cavity is very different from that of the 

 Cetacea, being small as compared with the size of the 

 animal, rather elongated and laterally compressed, truncated 

 at each end, and with the upper surface flattened. The 

 cerebellar fossa is large, and altogether behind the cerebral ; 

 the olfactory fossa is distinct, but small and narrow, bounded 

 on the inner side by a strongly-developed " crista galli " from 

 the mesethmoid. The foramen magnum is of great size ; its 

 plane looks backwards and downwards. The supraoccipital 

 (SO) is inclined forwards, but does not extend beyond the 

 ridge bounding the occipital region ; the roof of the cere- 

 bral fossa of the cranial cavity being formed by the parietals 

 (Pa). The upper surface of the skull is very narrow, and 

 flat or slightly arched in the longitudinal direction j its 

 sides, which are parallel for a considerable distance, join 

 at a right angle the vertical inner wall of the great temporal 

 fossa. The squamosal has an extremely massive and 

 long zygomatic process, flattened on the outer surface, and 

 posteriorly it sends down a strong triangular post-tympanic 

 process, articulating with a rough projecting edge of the 

 exoccipital. Above this, between the squamosal, supra- 

 occipital, and exoccipital, is a considerable vacuity in the 

 cranial wall, partly filled by the periotic (Per). The lower 



