332 



THE ANATOMY OF VERTEBRATED ANIMALS. 



Tlie premaxillary region of the palate, and the corresspond- 

 ino- surface of the mandible, are coated with mammillated and 

 rugose horny plates formed of hardened epithelium ; and, in 

 the extinct genus Hhytma^ these plates were the only masti- 

 cating organs, as there wiere no teeth. In Halicore (the 

 Dugong), there are teeth which have no vertical successors, 

 form no roots, and are devoid of enamel ; wliile, in 3Ianatus, 

 there are milk-molars, and the grinding teeth are enamelled^ 

 and present crowns with double transverse ridges. 



The adult Manatee has no incisors. In the Dugong there 

 are no incisors in the mandible of the adult. The male has 

 two tusk-like incisors w^hich project from their sockets in the 

 premaxillae; while, in the female, the tusks remain concealed 

 in their alveoli. 



In the foetal state, both Halicore and Manatus have in- 

 cisors in the mandible as well as in the premaxilia3. 



The stomach is divided into two portions by a median con- 

 striction, and its cardiac end is provided with a peculiar gland. 

 Its plyoric end, in some species, gives ofiP two C£8ca. 





Fig. 103. — Dorsal view of the heart of a Dugon? {ITaUcore\ its cavities being laid open. — R. 

 v., right ventricle; L. v., left ventricle. F. e. 8. s., left superior vena cava. V. c. s. d., 

 right superior vena cava. V. c. i., vena cava inferior. I", o. v., the inner end of a csecal 

 diverticulum of the right am-icle, into which a style is introduced, and which represents 

 the foramen ovale. , the auricular septum. 



There is a car-cum at the junction of the large and small in- 

 testine. Salivary glands are well developed. The apical por- 

 tion of the sex>tum ventriculorum is deeply cleft, so that the 

 ventricles are separated from one another through about half 

 their extent. 



Tliere are two superior cavae and a Eustachian valve. FJx' 



