506 CARNIVORES. 



basal tubercles, more or less developed ; in a few of the largest spe- 

 cies, they are simple and obtuse, and particularly so in the Walrus, 

 in which the molar teeth are reduced to a smaller number than in the 

 true Seals (1). In these the line of demarcation between the true and 

 false molars is very indefinitely indicated by characters of form or 

 position ; but, according to the instances in which a deciduous den- 

 tition has been observed, the first three permanent molars in both 

 jaws succeed and displace the same number of milk molars, and are 

 consequently premolars ; occasionally, in the Seals with two-rooted 

 molars, the more simple character of the premolar teeth is manifested 

 by their fangs being connate, and in the Stenorhynchus serridens the 

 more complex character of the true molars is manifested in the crown. 

 There is no special modification of the crown of any tooth by which it 

 can merit the name of a ' sectorial ;' but we may point with certainty 

 to the third molar above, and the fourth below, as answering to those 

 teeth which manifest the sectorial character in the terrestrial Car- 

 nivora. 



The coadaptation of the crowns of the upper and lower teeth is 

 more completely alternate than in any of the terrestrial Carnivora, 

 the lower tooth always passing into the interspace anterior to its 

 fellow^ in the upper jaw. In the genus Phoca proper {CalocephaluSy 

 Cuv.) typified by the common Seal {Ph. vitulina), the dental for- 

 mula is : 



Incisors — : canines — ; premolars — : molars — : = 34. 



2—2 ' 1_1 ' r 3_3 ' 2—2 



The forms and proportions of these teeth are shown in PI. 132, fig. 1. 

 The first tooth above and below presents a complete confluence 

 of the fangs ; they are separated in the rest above ; but below they 

 sometimes do not become free before the fourth, and sometimes 

 the two roots are distinct in the third and second molars. In the 

 Phoca anellata, Nills : the principal cusp of the molar teeth is com- 

 plicated with anterior and posterior smaller cusps, sometimes one 

 in number in the upper molars ; the anterior accessory cusp is some- 

 times wanting in the first, and is rudimentary in the rest ; but usually 



(1) The relation of Trickechus to the Phocidm is analogous to that of Machairodus to the 

 FelidcB, and also, in the simplification of the molars, to the relation of Pro teles to the Canidee. 



