568 



ZOOLOGY 



Whales, though teeth are developed in the foetal condition (Fig. 

 1195), they become lost either before or soon after birth, and they 

 are succeeded in the adult by the plates of baleen or whalebone 



(Fig. 1196), which, in the form of 

 numerous triangular plates, hang 

 vertically downwards from the palate. 

 Of the Sirenia, the Dugong and 

 Manatee have a hetei-odont denti- 

 tion ; in Ehytina teeth were absent. 

 In the two former Sirenians there 

 are incisors and molars with a wide 

 diastema between them. In the 

 Manatee there are two rudimentary 

 incisors on each side, both in the 

 upper and the lower jaw ; these dis- 

 appear before the adult condition is 

 reached. There are altogether eleven 

 molars on each side above and below, 

 but not more than six of these are 

 in use at once, the more anterior 

 when worn out being succeeded by 

 the more posterior. They have 

 enamelled crowns with transverse 

 ridges, and are preceded by milk- 

 teeth. In the Dugong there are no 

 incisors in the mandible of the 

 adult, and only one tusk-like pair 

 in the upper jaw, large in the male 

 — in which they grow from persistent 

 pulps — little developed in the female, 

 and remaining concealed in their sockets. In the young there 

 are rudimentary incisors in the mandible, and also a rudimentary 

 second pair in the upper jaw. There are either five or six molars 

 on each side, both in the upper and lower jaws. These are 

 cylindrical teeth, devoid of enamel, and with persistent pulps. 



In the Carnivora vera (Fig. 1197) the dentition is complete, 

 heterodont and diphyodont, and all the teeth are provided with 

 roots. The incisors are relatively small, chisel-shaped teeth ; there 

 are nearly always three of them on each side, in both upper and 

 lower jaws. The canines are always large and pointed. The 

 presence of carnassials, consisting of the last pre-molar in the upper, 

 and the first molar in the lower, jaw, is universal. In front of the 

 carnassial the teeth are compressed and pointed ; behind it they 

 have broad surfaces. In the Cat family {Felidce) the formula is — 



Fig. 1196. — Section of upper jaw, with 

 baleen'plates, of Balsenoptera. 



(After Owen.) 



. 3 1 S 1 



3 1 -^ 2 1 



30. 



