TEETH OF DIPHYODONTS. 283 



entirely closed in a bony cell, in which respect the Cetacea offer 

 an interesting analogy to true fishes. 



§ 220. Teeth of Diphyodonts. A. Szrenia. — Two marks of 

 inferiority in the dental system of the carnivorous Cetacea, which 

 they have in common with many of the order Bruta, viz. a general 

 uniformity of shape in the whole series of teeth, and no succession 

 and displacement by a second or permanent set, disappear when we 

 commence the examination of the dentition of those apodal pachy- 

 derms which were called by Cuvier the Herbivorous Cetacea. 



In the Dugong (Halicore), for example, we find incisors dis- 

 tinguished by their configuration as well as position from the 

 molars, and the incisive tusk is deciduous, displaced vertically, 

 and succeeded by a permanent tusk ; both these characters are 

 shown in fig. 160, vol. ii. p. 281. Of the incisors of the Dugong, 

 only the superior ones project from the gum in the male sex, and 

 neither upper nor lower ones are visible in the female. The supe- 

 rior incisors, ib. i, are two in number in both sexes. In the male 

 they are moderately long, subtriedral, of the same diameter from 

 the base to near the apex, which is obliquely bevelled off to a 

 sharp edge, like the scalpriform teeth of the Rodentia. Only the 

 extremity of this tusk projects from the jaw, at least seven-eighths 

 of its extent being lodged in the socket, the parietes of which are 

 entire. In the female Dugong the growth of the permanent inci- 

 sive tusks of the upper jaw is arrested before they cut the gum, 

 and they remain throughout life concealed in the premaxillary 

 bones ; the tusk in this sex is solid, is about an inch shorter and 

 less bent than that of the male ; it is also irregularly cylindrical, 

 longitudinally indented, and it gradually diminishes to an obtuse 

 rugged point; the base is suddenly expanded, bent obliquely 

 outwards, and presents a shallow excavation. The deciduous 

 incisors of the upper jaw, i, d, are much smaller than the perma- 

 nent tusks of the female, and are loosely inserted by one extremity 

 in conical sockets immediately anterior to those of the permanent 

 tusks, adhering by their opposite ends to their tegumentary gum, 

 which presents no outward indication of their presence. Not more 

 than twenty-four molar teeth are developed in the Australian 

 Dugong {Halicore Australis), or more than twenty molar teeth in 

 the Malayan Dugong, viz., in the latter, five on each side of both 

 upper and lower jaws, ib. 1-5, but these are never simultaneously 

 in use, the first being shed before the last has cut the gum. 



The molar teeth of the Dugong consist of a large body of 

 dentine, a small central part of osteo-dentine, and a thick external 

 investment of cement, c, fig. 242, vol. i. p. 365. In the female 



