166 Bulletin American Museum of Naturcd History. [Vol. XX^'II, 



that the upper dentition of Plagiaulax must have conformed to the 

 BoJodon type. 



Affinities of Tritylodon and Microlestes. 



Aside from the American Triassic genera Dromatherium. and Micro- 

 conodon (p. 163), which can scarcely be considered full-fledged mammals 

 (p. 164), the oldest known mammals are Tritylodon Jongoevus Owen, founded 

 on the front part of a skvdl probably from the Lower Jurassic (Broom, 

 1909.3, p. 289) of South Africa, and the genus Microlestes from the Rhfetic 

 of Germany and England. Broom (1903) has recently described the jaw 

 of a small animal (Karooniys), from the Upper Triassic of South Africa, 

 which may possibly have belonged to a mammal. 



Tritylodon was regarded as a mammal by Owen, Osborn and Lydekker 

 and more recently by Broom, but Seeley (1895.2, pp. 1025-1028) treated it 

 as a relative of the Gomphodont reptiles. There does not appear to be very 

 convincing evidence for this allocation. Tritylodon does indeed somewhat 

 resemble Trirachodon in the top view of the snout but Owen's figures reveal 

 no evidence of a postorbito-frontal bar on either side, a character which 

 might be expected to be present in a supposed relative of Gomphagnathus. 

 On the other hand the wide post-incisive diastema of Tritylodon appears to 

 be lacking in the Gomphognaths in which also the opposite molar rows curve 

 inward and converge anteriorly. In Tritylodon they are straight and parallel. 

 In the known Gomphognaths the upper cheek teeth consist of transverse 

 ovals, often with an irregular transverse ridge. In Tritylodon the molars 

 bear three longitudinal rows of small tubercles, an arrangement known else- 

 where only in Multituberculates. 



Tritylodon is either a Midtituberculate or offers a very close convergent 

 resemblance to that general type. Its resemblances to Multituberculates 

 are very many in proportion to the total number of known characters. The 

 up])er incisors, judging from their relative ])osition (which recalls the condi- 

 tions in Duplicidentate Rodents) and in connection with the Multitubercu- 

 late pattern of the molars, may well have been of the peculiar prehensile- 

 incisive type seen in the Upper Jurassic AUoelon Marsh. The molars are 

 somewhat like those of Meniscoessus of the American Upper Cretaceous, 

 but the small cusps, arranged in three longitudinal rows, are rounded and not 

 sub-crescentic. The hard palate apparently extends well backward. The 

 excursion of the mandible was probably from in front upward and some- 

 what backward but the backward motion could hardly have been as extreme 

 as in Castor, otherwise the small tubercles would have been worn down level 

 with the crown. But even a slight backward movement of the mandible 



