No. 377.] THE ORIGIN OF THE MAMMALIA. 327 
donts. The homologies of this tooth in Tritylodon are uncertain. 
The premolar teeth are small and circular, usually tuberculate, 
but occasionally the first tooth is compressed laterally. The 
D. browni, 
Tritylodon 
Fic. 12. — Dental and cranial structures of the Gomphodontia. D. masticus, analogous to a 
lo cy Dd. poker trituberculate structure more 
obscure, D. brachytizra, resembling the lower molars of Microlestes from the Rhætic 
of Germany, showing ge incipient division of the liat The skull of Tritylodon is 
greatly reduced. (After 
molar teeth are usually single rooted, arranged in close-set 
series, which diverge sharply outwards as they extend back- 
wards; with grinding surfaces varying in form and character, 
but with internal and external cusps more prominent than the 
