332 THE ANATOMY OF VERTEBRATED ANIMALS. 



Tlie premaxillary region of the palate, and the correspond- 

 ing surface of the mandible, are coated with mammillated and 

 rugose horny plates formed of hardened epithelium ; and, in 

 the extinct genus jRhytina, these plates were the only masti-, 

 eating organs, as there were no teeth. In Salicore (the 

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

 form no roots, and are devoid of enamel; while, in Manatus, 

 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 which project from their sockets in the 

 premaxill* ; while, in the female, the tusks remain concealed 

 in their alveoli. 



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

 cisors in the mandible as well as in the premaxiliae. 



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 off two caeca. 



Fig. 103. — Dorsal view of tbc heart of a Dugong {UaUcors), its cjivities beinj? laid open. — R 

 v.. right VL-ntricIe; L. v., left ventricle. V, c. s. ft,, left superior vena cava. V. c. s. d.. 

 right su]ierior vona cava. V. c. i., vena cava inferior. F. o. r.. the inner end of a csecai 

 diverticulum of the right auricls, into which a style is introduced, and which represents 

 the foramen ovale. , the auricular septum. 



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

 testine. Salivary glands are well developed. The apical pop> 

 tion of the septum ventriculorum is deeply cleft, so that the 

 ventricles are separated from one another through about half 

 their extent. 



There are two superior cavae and a Eustachian valve. Ex- 



