1885.] DR. ST. G. MIVART ON THE PINNIPEDIA. 495 



of the bulla descends more and more mesiad, culminating in a ridge 

 which is just external to this imperfect ossification. The lower lip 

 of the meatus auditorius externus is not produced outwards ; the 

 mastoid process extends much further out and the meatus opens 

 rather downwards. There is a very large paroccipital process, which 

 is bent back and joins the very large mastoid process by a continuous 

 undulating ridge, or wall, of bone. 



The stylo-mastoid foramen is large, and not, as in the Seals, 

 situated in a narrow groove between the mastoid and tympanic. 

 There is a small postglenoid foramen. The palatine foramina are 

 placed in the anterior half of the palate. There are great defects 

 of ossification in the region of the spheno-palatine foramen. The basis 

 cranii is curved, convex downwards, antero-posteriorly as in Phoca, 

 but it is sharper and not so rounded. The alisphenoid is joined by a 

 pointed prolongation of the parietal. There are small pterygoid 

 fossae and long hamular processes. There is an alisphenoid canal. 

 There is a small or large preorbital process. A venous channel in 

 the exoccipital opens inside the condyle. The condyloid foramen is 

 larger than in the Seals. 



The hinder part of the palate may be very deeply concave. The 

 optic foramen opens singly into the cranial cavity. The cerebellar 

 fossa of the petrosal seems generally very small. The premaxilla 

 may develop a median process above the incisors. Besides 

 enormous sagittal and lambdoidal ridges, there may be processes 

 developed from the side of the skull like parts of a ridge extendino- 

 backwards and downwards from the hinder part of the frontal to the 

 lambdoidal ridge. 



The mandible may have no subangular process or a small one, but 

 there is a large "angle " very near the condyle and inflected as much 

 as in any marsupial. 



Dentition :— I. |, C. \, P. f, M. ^^, =34 or 36. 



The molars have mostly but one root and a crown, which 

 would be conical but that it is more or less compressed, with a 

 cingulum whence more or less of an anterior and posterior accessory 

 cusp may be developed. The outer incisors are rather large and 

 shaped like canines. The other incisors are each divided "at the 

 summit into two pretty equal cusps by a transverse groove. 



Otaria and Trichechus must be accepted as representing two 

 groups of about the same value as that which includes the remaining 

 genera. Thus we have the arrangement already put forward by 

 Mr. Turner and Professor Flower, which may be shortly tabulated 

 as follows : — 



Pinnipeds with external ears and an ali- 

 sphenoid canal OTARIlDJi;. 



Otaria. 

 Without external ears, but with an alisphe- 

 noid canal TRTCHECHIDiE. 



Trichechus. 



