XIII 



PHYLUM CHORDATA 



567 



In the Hyracoidea the dental formula is 

 .10 4 3 

 *-2' C '0' |7 -4>-3 =34 - 



The upper incisors are not unlike the larger pair of the Rabbit in 

 shape, though prismatic and pointed, instead of compressed and 

 chisel-like ; they grow from persistent pulps. The outer incisors 

 are elongated, inclined forwards, and trilobed at the extremities. 

 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 Perigsodactyla. 



The Elephants have an extremely specialised dentition. There 

 are no canines and no lower incisors. The single pair of upper 

 incisors are developed into the enormous tusks (Fig. 1181, /), 

 which grow continuously from persistent pulps throughout the 



FIG. 1194. Grinding surface of a partially worn right upper molar of the African Elephant 

 (Elephas africanus). (After Owen.) 



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. 1194) 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. 



Fia. 1195. Left lower jaw of foetus of Balaenoptera ro strata, inner aspect, showing teeth ; 

 natural size. (After Julin.) 



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 Whalebone 



