THE SKELETON IN MAMMALIA. THE TEETH. 425 



preceded by two-rooted milk teeth. The Aard Varks are 

 diphyodont, and milk teeth are also known in a species of 

 Dasypus, but with these exceptions Edentates are, as far as is 

 known, monophyodont. In Glyptodon the teeth are almost 

 divided into three lobes by two deep grooves on each side. 



The Aard Varks (Orycteropodidae) are quite exceptional 

 as regards their teeth, which are cylindrical in shape, and 

 are made up of a number of elongated denticles fused to- 

 gether. Each denticle contains a pulp cavity from which a 

 number of minute tubes radiate outwards. These teeth are 

 diphyodont and somewhat heterodont, eight to ten pairs occur 

 in the upper jaw and eight in the lower, but they are not all in 

 place at one time. The last three teeth in each jaw are not 

 preceded by milk teeth 1 . 



SIRENIA. The teeth of Sirenia show several very distinct 

 types, the least modified being that of the extinct Hali- 

 theriidae, which have large incisors in the upper jaw, and five 

 or six pairs of tuberculated grinding teeth in each jaw, the 

 anterior ones being preceded by milk teeth. 



In both the living genera the dentition is monophyodont. 



9 11 



In Afanatus the dentition is i -^ pm and m . The incisors are 

 , 2 



vestigial, and disappear before maturity. The grinding teeth 

 have square enamelled crowns marked by transverse tuber- 

 culated ridges. They are not all present in the jaw at the same 

 time. In Halicore the upper jaw bears a pair of straight tusk- 

 like incisors ; in the male these have persistent pulps and 

 project out of the mouth; in the female they soon cease to 

 grow and are never cut. They are separated by a long 

 diastema from the grinding teeth which have tuberculated 

 crowns and are 4- or in number, but are not all in place at 

 once. Several other pairs of slender teeth occur in the young 

 animal, but are absorbed or fall out before maturity. In 

 Rhytina teeth are altogether absent. 



1 See Oldfield Thomas, P. R. S. t vol. XLVII., p. 246 (1890). 



