^2 Southern Cross. 



species. The only southern Seal which at all nearly approaches 

 Ommatophoca in the size of the teeth is Lcptoni/chotes ; but here 

 again the canines of the latter species arc many times larger than 

 those of the former, while the cheek-teeth are also larger and of a 

 different shape. In Ommatophoca the cheek-teeth are provided with 

 three cusps, the central one being the largest. In Leptonychotes 

 the rudiments of posterior or anterior cusps, even when present, 

 are obscured by the prominence of the central cusp. In spite 

 of their feebleness, the teeth of young specimens of Ommatoplwca 

 may, as is usual with other Pinnipeds, be so crowded in the jaw as 

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

 their roots are embedded, the space between them increases, and the 

 position of their long axis becomes parallel to that of the jaw itself. 



The second point in connection with the teeth of Ross's Seal is 

 the remarkable 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 brought home by Eoss, and 

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

 thal.^ The difficulties and interest of the subject have been doubly 

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

 ' Southern Cross.' Before dealing ^vith these it will be well to describe 

 the peculiarities of the teeth of Eoss's specimens. 



In one of these skulls (No. 43 . 11 . 25 . 4) the single-rooted first post- 

 canine 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 



4x4 1x1 



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



4x41x1 



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

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

 the upper 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 represent the first cheek-tooth of No. 43.11.25.4, since the 

 corresponding tooth on the right side, although single, is provided 

 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 

 teeth, these have apparently originated from some kind of splitting 

 process, since on the right side this process has been imperfectly 

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



^ P. Z. S. 1892, pp. 106-108 ; also ' Materials for Study of Variation,' pp. 237- 

 238, 1894. 



2 Jena. Zeitsclir. (2), xxviii. pji. 76-118, pi. III., IV. 1893. 



