XIII 



PHYLUM CHORDATA 



535 



The pre-molars and molars form a continuous series, separated by 

 an interval from the incisors, and in pattern closely resemble those 

 of some of the Perissodactyla. 



The Elephants (Fig. 1125) have an extremely specialized denti- 

 tion. There are no canines and no lower incisors. The single 



FIG. 1125. Grinding surface of a partially worn right upper molar of the African Elephant 



ajricanus). (After Owen.) 



pair of upper incisors are developed into the enormous tusks 

 (Fig. 1112, /), which grow continuously from persistent pulps 

 throughout the life of the animal ; they are 'of elongated conical 

 form, and usually become curved. The tusks are composed of 

 solid dentine, enamel occurring only on the apices, and becoming 

 early worn away. The molars (Fig. 1125) are very large, and 

 their worn surfaces are marked with prominent transverse ridges ; 

 there are six molars altogether on each side, but only one or two 

 are functional at once, the more posterior moving forward and 

 taking the place of the more anterior as these become worn out. 



When teeth are developed in the Cetacea they are nearly 

 always numerous, homodont, and monophyodont ; in the Sperm- 

 whales they are confined to the lower jaw. In the Whale-bone 



FIG. 1126. Left lower jaw of foetus of Balsenoptera rostrata, inner aspect, size ; showing 

 teeth, natural aspect. (After Julin.) 



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

 1126), they become lost either before or soon after birth, and their 

 place is taken in the adult by the plates of baleen or whalebone 

 (Fig. 1127), which, in the form of numerous triangular plates, hang 

 vertically downwards from the palate. 



Of the Sirenia, the Dugong and Manatee have a heterodont 



