9 6 



GIDLEY 



tors of the mammals was found upon the antero-internal side or 

 on the antero-external side of the upper molars." This evidence, 

 according to Osborn, is in favor of the tritubercular hypothesis, 

 and conclusive evidence of the theory is furnished in the Jurassic 

 mammal molars. However, a study of all the mesozoic mam- 

 mal material available has led the present writer to exactly 

 opposite conclusions. 



Unfortunately, Osborn's observations were confined to a very 

 limited amount of material, and from a careful examination of 

 the teeth of Triconodon and Dryolestcs, 1 two forms especially 

 studied by him, it seems that his conclusions were based on 

 evident, though perfectly excusable, errors of observation, due 

 doubtless to the minuteness of the teeth and their dark color, 

 which make it difficult in many cases to distinguish, between a 

 fracture and the natural surface of the tooth. Thus, according 

 to Osborn, 2 the upper molars of Dryolestes are " broadly trans- 

 verse or triangular and upon the internal side of each is a large, 

 conical, pointed cusp,/r, supported by a large stout fang, . . . 

 The external portion of the crown is depressed, and bears one 

 large antero-external cusp ? pa and one smaller postero-external 

 cusp ? me which is either partially worn away or less pronounced 

 in development." But there are two important cusps not noted by 

 Osborn, one an external cusp placed anterior to the main external 

 cusp, the other a small but well-defined intermediate cusp appear- 

 ing on the posterior transverse ridge. Thus there are five distinct 

 cusps instead of three, as stated by Osborn, and these do not form 

 a trigon in the sense that this term has been used, for the main 

 external cusp is in the middle of the base of the triangle instead 

 of forming one of its angles. 



In the upper molars of Triconodon the three principal cusps are 

 arranged in a direct line, and are nearly equal in size and form, 

 and the two lateral cones are each supplemented by a small but 

 well-defined internal basal heel-like cusp and an external basal 

 cingulum. The main cusps are flattened externally into a con- 

 tinuous wall in one species (see PL V, fig. i), while they are 



1 The specimens studied by the present writer and referred to these genera 

 are from the Atlantasaurus beds of Wyoming. These beds are usually referred 

 to the upper Jurassic, although they may be lower Cretaceous. 



2 Amer. Journ. Science (4), Vol. 17, 1904, 322. 



