j:;- MKinsTic VARIATION. [PARTI. 



a posterior halt'. The cusp of each tooth is also divided by the 

 grooves -. as to form two small cusps. Each of these teeth is 

 theref-re an imperfectly double structure, and maybe described as 

 bein : _r ju-t half- way between a single tooth and two teeth. These 

 are shewn in Fig. 56. 



l'ii,. ."(>. (iiniiHitojihnca ros.iii. No. 320. The anterior premolars of upper and 

 ]n\viT jaws from the side. (The left lower and right upper teeth were not 



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 

 -imilar 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 denial formula of this skull, taken as it stands, is therefore 



5 4 1 1 . 



[I . 111 , tor since the bigemmous teeth are not com- 



*T - T? 1 - 1 



jiletely divided, they must be reckoned as single teeth. 



:\-l\. Cystophora cristata : internal to and slightly in front of// on 

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

 alike in t'i>nii but are rather smaller than P^. C. M., 895. 



;;o-> Cystophora cristata (label, Phoca cristata): internal to ri;//tt 

 upper ," ! is an alveolus for a small one-rooted tooth. In the corre- 

 sponding situation in the left /<>/>,,- jaw there is such an extra tooth in 



place. /,'//'/. J/. 



Zalophus lobatus ( = 0tdi-i>i lultot,,)-. left ^ smaller than right 

 7* 1 , and between the canine and the left i^_ there is a supernumerary 

 tooth, smaller than left /<'. (The same skull has another extra tooth 

 outside and between jr' and j" 4 , see below No. 333.) Leyd. ^f. 



[P. vitulina: nlv.-nlus tur left /'' much larger than that for rt. P l ; the latter 

 tootli is in place, but Irft /' is missing. C. M., IIU'2.] 



Lur'ie Prenwlars. 



P. groenlandica : in the position in which left upper ;? 4 should 

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

 l>\ Fig. ""). The two stand perfectly in series, and owing to the wide 



