xii.] SIRENIA. 223 



cylindrical and tapering, in B. rostrata, flat and pointed 

 externally. They always ankylose with the basihyal. 



Order SIRENIA. The animals belonging to this order, 

 restricted at the present time to only two genera, which were 

 formerly, but quite erroneously, included among the Cetacea, 

 have skulls constructed on a very peculiar type, though with 

 some affinities both to the Ungulata and the Proboscidea. 

 Many of the special modifications are adaptations to their 

 aquatic mode of life, and it is in these alone that they 

 present any resemblances to the Cetacea. 



The skull of the African Manatee (Manatus senegalensis, 

 Fig. 68), which may be taken as a type of the order, is remark- 

 able 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 



