io6 



AMERICAN MESOZOIC MAMMALIA 



YP-M- 1037s 



of the abraded surface of P4 in Meniscoessus, they represent a modification of the type 

 seen in other ptilodontid genera, and each is strongly worn on one side with fine 

 oblique striae due to shearing. They are, however, peculiar in detail and rather unlike 

 the homologous teeth in any other genera. Each of their several cusps is conical, 



pointed, with light crenulations radiat- 

 Y-P-M-to3/s ing from the apex. In the type of Oraco- 



donancefs (Y.P.M. No. 1 1862) the main 

 tip is eroded and the tooth is not as satis- 

 factory as in a referred specimen which 

 resembles it closely. In this tooth, Y.P.M. 

 No. 10378, the crown rises to an eccentric 

 apex which, from analogy with later gen- 

 era, is probably posterior. 1 n a row ante- 

 rior to this point and rapidly decreasing 

 progressively in height are three other 

 cusps. External to the second of these is 

 a small cusp of about the same size as the 

 anterior cusp of the main row. There is a 

 cuspule of like size posterointernal to the 

 apex, and another smaller one postero- 

 external to this point. Other specimens 

 differ chiefly in size. O. conulus is larger 

 and has an additional cuspule posterior 

 to the main point. These teeth are shorter 

 relative to M^ than in Ctenacodon or 

 Ptilodus, but the same is true of P4 rela- 

 tive to Ml. Furthermore it is probable 

 that the last upper premolar was shorter 

 than the last lower premolar, as is gen- 

 erally true in the Multituberculata. 



Granted that these teeth, Oracodon, 

 represent the last upper premolar of 

 Meniscoessus there remains the question 

 as to whether other upper premolars oc- 

 curred. In this connection it is important to remember (i) that all other multituber- 

 culates with lower premolars like those of Meniscoessus, some of them with the lower 

 premolars still more reduced, have three or four upper premolars, (2) that the only 

 genus known to have but one upper premolar {Taeniolabis) has the most reduced P4 

 in the order and is a very different adaptive type from Meniscoessus, and (3) that 

 Meniscoessus is apparently closely related to its contemporary Cimolomys and but lit- 

 tle more specialized, and Cimolomys is known to have had at least four premolars. A 

 ■priori then, one would expect Meniscoessus to have at least three upper premolars and 

 possibly more. 



AMMH. 17SS5 



Fig. 42. Multituberculates. Upper premolars. A, 

 Cimolomys, internal, crown, and external views of 

 last upper premolar. B, Meniscoessus {"Oraco- 

 don"), internal, crown, and external views of last 

 upper premolar. C, Ptilodus, crown view of upper 

 premolars. 



