1892.] 



NUMERICAL VARIATION IN TEETH. 



107 



is also divided by the grooves so as to form two small cusps. Each 

 of these teeth is therefore an imperfectly double structure, and may 

 be described as being just halfway between a single tooth and two 

 teeth. 



On the left side in the upper series, as the vis-d-vis to one of 

 these double teeth, there are two complete teeth, standing near 



Fig. 1. 



Ommafnphoca rossii, having the first upper premolar on the right side bigomi- 

 nous, and on the left side represented by two complete and similar 

 teeth. In the lower series the first premolar was bigeminous on both 

 sides. From a specimen in the British Museum. 



together, but having separate sockets divided from each other by a 

 bridge of bone. The dental formula for this skull taken as it stands 

 is : — i. ^z:2* ^' i^> P* + ™' szzjj ^^t since the bigeminous teeth are 

 not completely divided into two, they must be reckoned as single 

 teeth. 



Phoca grcenlandica. — A specimen is preserved in the Leyden Mu- 

 seum having the arrangement shown in the figure (fig. 2\ p. 108). 

 The dentition of the lower jaw is the same on both sides. In the 

 upper jaw there are on the left side six teeth behind the canine, 

 while on the right side there are ^ve, the normal number. But 

 upon examination, one of these teeth, namely p^ of the usual nomen- 

 clature, is an abnormally large tooth, being especially thick in a 

 transverse direction, and has besides a deep cleft in the crown, 



^ This figure was kindly drawn for me by Mr. J. J. Lisler. 



