52 



Soiitlicrti ( ^oss. 



sixTJcs. The »»iilv s«»ntlieni St';il wiiich ;it all nearly apimiaclies 

 (h,niut(<>p/i»cu ill tlio size "I" tin- irvXU is Lcploni/cholcii ; but here 

 a^niin llu- cjinines of the latter species aiv many times larger than 

 thijso of the former, while the cheek-teeth are also krger and of a 

 ililTen'nt shape. In ( hnmatophuca the cheek-teeth are provided with 

 throe rusp-s, the central one being the largest. In Lcptoaijchotes 

 the rudiment-s df po.sterior or anterior cusps, even when jjresent, 

 art' obsnuvil by the jjruminence of the central cusp. In spite 

 of their feebleness, the teeth of young specimens of Ommatophoca 

 may, as i.s usual with other Pinnipeds, be so crowded in the jaw as 

 to overlap one another. With the lengthening of the bone in which 

 their nxits an; embedded, the space between them increases, and the 

 |H)sitiou of their long axis becomes parallel to that of the jaw itself. 



The second \n>\\\i in connection with the teeth of Eoss's Seal is 

 the ivmarkalde instance of variation with which they present us. 

 This had attracted attention even when the species was represented 

 in collections only by the two skulls brouglit home by Eoss, and 

 formed the subject of discussion by Mr. Bateson ^ and Dr. Kiiken- 

 tlial.^ The dilticulties and interest of the subject have been doubly 

 increased by the specimens brought home by the 'BcUjica ' and the 

 ' Soutfuni Cross.' Before dealing with these it will be well to describe 

 the pocidiarities of the teeth of Eoss's specimens. 



In one of these skulls (No. 4o . 11 . 25 .4) the single-rooted first post- 

 cJinine is followed on each side of either jaw' by three double-rooted pre- 

 molars and one true molar, that is to say, if, taking the analogy from 



other Seal.-., we regard the dental formula as p.m. — ^— m. 



4x4 1x1' 



This dentition has been supposed by Mr. Bateson to represent the 



normal arrangement in the animal. In No. 324&, on the contrary, 



the uj.iKir jaw is provided with six cheek-teeth on the left side, the 



first two of which are smaller, and it is reasonable to suppose that 



they repre.sent the fii-st cheek-tooth of No. 43.11.25.4, since the 



c<irre.siM.nding tooth on the right side, although single, is in-ovided 



with two distinct crowns, and looks, when in situ, exactly like two 



teeth united in a single cavity. Presumably then, although the 



first cheek-tooth of the left side is represented by two distinct 



U-eth, these have apparently originated from some kind of splitting 



\>t^>cem, since on the right side this process has been imperfectly 



carrietl out, leaving an organ which is exactly half-way between 



03«' y."^ ^- ^®^-' ^1'- 10G-10« ; iilso ' Materials for Study of Variation,' pp. 237- 

 /eiucbr. (*.'), xxviii. pp. 70-118, pi. 111., IV. 1893. 



