40 



AMERICAN MESOZOIC MAMMALIA 



Marsh based this species on (i) its supposedly smaller teeth, (2) the inflected 

 inner margin with angle extending down below condyle, and (3) the lack of elevation 

 of the condyle above the level of the alveolar border. The first character must have been 

 cited inadvertently, for Marsh's own measurements show that the teeth are of exactly 

 the same size as in T. bellus. The second and third characters are due entirely to the 

 broken and crushed nature of the specimen and to the difficulty of orienting it. The 

 only differences of any importance from T. bellus are those given in the diagnosis, and 

 it is by no means certain that these are not due to some cause other than real specific 

 distinction. In Marsh's descriptions of Menacodon and Tinodon the only directly 

 comparable character is the relative vagueness of the internal groove in the former. 

 This is not of specific or greater value, and there can be no doubt that Menacodon rams 

 is synonymous with Tinodon lefidus. 



AMPHIDONTIDAE Simpson 1925 



Definition. — Lower molars functionally unicuspid. Accessory cuspules mere 

 crenulations in sharp cutting edges of main cusp. No continuous cingulum, but ante- 

 rior and posterior inner basal projections, which may form small cusps. 



Type. — Amfhidon Simpson. 



Distribution. — Upper Jurassic of Wyoming. 



This family rests on a single specimen, but the characters of this specimen are so 

 clearly displayed and so extraordinary that its right to this distinction can hardly be 

 questioned. Teeth resembling these molars, even by convergence, are not known in any 

 other animal, living or extinct. The reference of the Amphidontidae to the Symmetro- 

 donta is not certain, but it seems probable. The mandible, so far as known, seems to be 

 of symmetrodont t)^e, the premolars are similar, and the molars, while very different 

 in detail from those of S-palacotherium or Tinodon, seem to have a certain basic resem- 

 blance. Certainly there is no other order as yet established to which the Amphidontidae 

 could be reasonably referred. 



Fig. 17. Amphidon superstes. Above, right last premolar and molars, in- 

 ternal view, type. Below, restoration of Mi, internal and crown views. 

 Fifteen times natural size. 



