336 



ANATOMY OF VERTEBRATES. 



264 



22 



Dentition of Seal (Phoca). 



A tendency to deviate from the ferine number of the incisors 

 is seen in the most aquatic and piscivorous of the Musteline 

 quadrupeds, viz., the Sea-otter (JEnhydra), in which species the 

 two middle incisors of the lower jaw are not developed in the 

 permanent dentition. In the family of true Seals the incisive 

 formula is further reduced, in some species even to zero in the 

 lower jaw, and it never exceeds f :- . All the PhocidcE possess 

 powerful canines ; only in the aberrant Walrus, fig. 265, are 

 they absent in the lower jaw, but this is compensated by the 

 singular excess of development which they manifest in the upper 



jaw. The molar series, fig. 

 264, m, usually includes five, 

 rarely six, teeth on each side 

 of the upper jaw, and five 

 on each side of the lower 

 jaw ; with crowns which vary 

 little in size or form in the 

 same individual. They are 

 supported in some genera, as 

 the Eared Seals ( Otarice) 

 and Elephant Seals ( Cystophord), by a single fang ; in other 

 genera by two fangs, which are usually connate in the first 

 or second teeth ; the fang or fangs of both incisors, canines, 

 and molars, are always remarkable for their thickness, which 

 commonly surpasses the longest diameter of the crown. The 

 crowns are most commonly compressed, conical, more or less 

 pointed, with the ( cingulum ' and the anterior and posterior 

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

 species 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. 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 dentition has been observed, the first three 

 permanent molars in both jaws succeed and displace the same 

 number of milk-molars, and are consequently, ' premolars ; ' occa- 

 sionally, 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 Stenorhy nchus 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 ' or f carnassial ; ' but we may 

 point with certainty to the third inolar above and the fourth 



