OCR Text |
Show 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 extending 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. % M . 1-^, = 3 4 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 alisphenoid canal OTARIlDiE. Otaria. Without external ears, but with an alisphenoid canal TRICHECHIDiE. Trichechus. |