238 



MERISTIC VARIATION. 



[part I. 



a posterior half. The cusp of each tooth 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 half-way between a single tooth and two teeth. These 

 teeth are shewn in Fig. 56. 



Fig. 56. Ommatophoca rosxii, No. 320. The anterior premolars of upper and 

 lower jaws from the side. (The left lower and right upper teeth were not 

 extracted.) 



On the left side in the upper jaw, as the vis-a-vis to one of 



these double teeth, there are actually two complete teeth, of very 



similar but not identical form, as shewn in Fig. 56. Each stands 



in a distinct alveolus, the two being separated by a bridge of bone. 



The dental formula of this skull, taken as it stands, is therefore 



5 4, i \ 



P \ 7>in>^ 7 > for since the bigeminous teeth are not com- 



r 4 — 4 1 — 1 & 



pletely divided, they must be reckoned as single teeth. 



321. Cystophora cristata : internal to and slightly in front of />' on 



each side in the upper jaws is an extra tooth. These extra teeth are 

 alike in form but are rather smaller than p 1 . C. M., 895. 



322. Cystophora cristata (label, Phoca cristata): internal to right 

 upper p 1 is an alveolus for a small one-rooted tooth. In the corre- 

 sponding situation in the left lower jaw there is such an extra tooth in. 

 place. Leyd. M. 



323. Zalophus lobatus ( = Otaria lobata): left P^ smaller than right 

 P^, and between the canine and the left p 1 there is a supernumerary 

 tooth, smaller than left p\ (The same skull has another extra tooth 

 outside and between p^ and p* see below No. 333.) Leyd. M. 



[P. vitulina: alveolus for left p\ much larger than that for rt. p l ; the latter 

 tooth is in place, but left p| is missing. C. M., 902.] 



Large Premolars. 



324. J?« grcenlandica : in the position in which left upper p* should 

 stand there are two whole and complete teeth, each as large as normal 

 P^. Fig. 57). The two stand perfectly in series, and owing to the wide 



