AKT. 15 ANATOMY OF THE EARED AND EARLESS SEALS HOWELL 19 



cinerea) of 82 mm. the overthrust amounts to 5 mm. A juvenal skull 

 of a Gallorhinus alascanus of 134 mm. exhibits this condition to an 

 extent as great as 25 mm. It was therefore surprising to encounter 

 in a large immature of Phoca vitulina with a skull length of 145 mm. 

 an overthrust amounting to but 8 mm., and this only for 10 mm. 

 upon either side of the sagittal line, laterad to that point it being but 

 a couple of millimeters. Both the otariid and the phocid presum- 

 ably have encountered, during their aquatic existence, the same or a 

 very similar sort of mechanical stimulus exerted by the water upon 

 the head. It is therefore necessary that the great difference in the 

 amount of this osteological overthrust exhibited by the supraoccipital 

 be accorded phylogenetic significance of great weight. 



The form of the mandible is chiefly influenced by that of the skull 

 proper. To conform to the heavier dentition and musculature of 

 ZalopJms, its mandible is stouter than that of Phoca hispida. The 

 coronoid process is very much broader and more suited to the inser- 

 tion of a large temporal muscle, but relatively the masseteric fossa 

 is about the same in both. The insertion of the superficial part of 

 the masseter is horizontal in Zalophus and at an angle of about 45° 

 in the Phoca, which variation is at least partly mechanical and due 

 to the fact that in the former the mandible is almost straight, while 

 in the latter it is much curved. This condition has been brought 

 about by the position in the Phoca of the glenoid fossa, situated well 

 dorsad of the maxillary tooth row, causing a corresponding position 

 of the mandibular condyle well dorsad of the mandibular tooth row. 

 There is much specific and generic variation exhibited by the mandi- 

 bles of the two families, however. 



There is available no adult pinniped skeleton in which the hyoid 

 complex is entirely satisfactory. It is, however, of the usual carni- 

 vore type, with basihyal, lesser cornua consisting of short thyrohyal 

 (no chondrohyal was noted), and greater cornua. The latter com- 

 prise ceratohyals, adjoining the basihyal, and then in sequence 

 epihyals, stylohyals, and tympanohyal elements, although it is not 

 certain if the latter are completely ossified in all pinnipeds. 



VERTEBRAL COLUMN 



As previously mentioned, the sum of the thoracic, lumbar, and 

 sacral series of vertebrae is used as a standard with which to compare 

 each series. Unfortunately, no account can be taken in the cleaned 

 skeleton of the thickness of the intervertebral disks; but after all, 

 what is desired is just so7)ie standard for comparison. The column 



