SEALS 5 



Remarks : — Owing to the immaturity of the ô spécimen (no. 700), the two skulls collected 

 by the Belgica furnish no évidence that the sexes, when adult, are distinguishable either by the 

 size of the teeth, by that of the whole skull or by the developement of the sagittal or coronoid 

 crests ; and the two spécimens in the British Muséum are so similar to them that I hâve been 

 unable to détermine their sex. Of the former the larger skull (no. 897) is considerably finer than 

 either of Ross' spécimens : the lesser skull (no. 700) is not adult and is the smallest of the four. 

 The différences of size represent individuals of différent âges. As is usual in the case of other 

 Pinnipeds the cheek-teeth of immature spécimens may be crowded or may over-leap each other : 

 the space between them increases and the position of their long axis relatively to the jaw itself 

 alters with the lenç;theninçr of the bones in which their roots are embedded. 



Apart from the teeth (to be considered separately) I find little individual variation in 

 the skulls of Ommatophoca. The most variable character (apart from the dentition) seems to be 

 the amount of the superior maxilla which enters into the boundary of the superior nares. The 

 length of the section of the boundary formed by one of thèse bones varies in the différent spéci- 

 mens from g to 17 millimètres. 



Dentition : — The two skulls of this seal, brought home by Ross, and presedved in the 

 British Muséum afford a most remarkable instance of variation in dentition, as first described by 

 Gray, (') Bateson and ( 2 ) D' Kukenthal. 



In one of thèse skulls (no. 43-1 1-25-4) tne nrst cheek-tooth is single rooted : the remaining 

 four are double-rooted. Taking the analogy from other seals we may regard the dental formula 

 as pm 3-^ m. ^-rp-, a combination which is supposed to represent the normal arrangement in the 

 species. In no. 324 b, 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. 43-1 1-25-4, 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. It would seem then that, whereas the first cheek-tooth of the left side is 

 represented by two distinct teeth they would appear to hâve owed their origin to 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 one and two teeth, being in fact a single root with 

 two crowns. The remaining teeth of the séries differ markedly from the corresponding teeth of 

 no. 43-1 1-25-4 m tna -t only one of them is distinctly double-rooted. The bifurcation of the root is 

 represented in the remainder merely by a slight basai notch, most prominent in the last tooth 

 on the left side, and by a central groove which passing up each side of the root and being deeper 

 externally evidently represents the double rooting of the cheek-teeth of no. 43-1 1-25-4. The groove 

 is so prominent in the last tooth on the right side that the tooth is actually double rooted, 

 although not so markedly so as in the corresponding tooth of no. 43-1 1-25-4. 



The teeth of the lower jaw represent in various degrees, the steps between single and 

 double-rooted teeth. Each of the anterior pair is double-crowned, but single-rooted, like the first 

 tooth on the right side of the upper jaw. The second and third teeth on each side possess a 

 broad, compressed, single root, divided by a rather deep, central, longitudinal groove on each 



(1) See : Proc. Zool. Soc, 1892, pp. 105-107 and 114-115. and : « Materials for the Study of Variation » etc., 

 London and New York, pp. 237-23S, 1894. 



(2) In : Jenaische Zeitschrift fur Naturwissenschaft, xxvin Bd. [N. F. xxi], p. n5, i8g3. 



