xii.] SIRENIA. 227 



The two last-named bones are ankylosed together, but not 

 to any of the other bones of the skull, and though freely 

 movable in the dried skull, they are retained in their place 

 by the overhanging process of the squamosal. 



The tympanic (Ty) consists of a large and very solid half- 

 ring, with its lower margin considerably thickened and 

 produced downwards ; but not forming any bulla, or any 

 tubular meatus. It is only attached to the periotic by its 

 extremities, and close to the inner side of its posterior 

 attachment is a well-marked tympanohyal ankylosed to the 

 periotic. 



The periotic (Per) is large and rounded externally. It 

 forms a very considerable part of the inner wall of the 

 cranium. Besides the portion containing the organ of hear- 

 ing, it has a large solid upper part of somewhat kidney 

 shape, lying in a groove in the squamosal (Sq). This solid 

 part has a large anterior prominence, to which the anterior 

 limb of the tympanic ring is ankylosed, and a smaller rounded 

 posterior projection, corresponding with the mastoid of other 

 Mammals, and mentioned before as appearing on the ex- 

 ternal surface of the skull, in the vacuity between the supra- 

 occipital, exoccipital, and squamosal. 



The foramina at the base of the skull are very few and 

 simple, as nearly all the nerves appear to pass out only by 

 the rather small sphenoidal fissure, and by the great confluent 

 median and posterior foramina lacera. There is a small 

 optic foramen passing through the middle of the orbito- 

 sphenoid, but the alisphenoid is imperforate, and even the 

 condylar foramen in the exoccipital for the hypoglossal 

 nerve, so constant in all Mammals (except the Elephant), is 

 represented by a groove (in some instances with a narrow 

 bridge across it) on the anterior edge of the bone. There 

 is no distinct carotid canal. 



The mandible is exceedingly different from that of the 



Q 2 



