432 PROCEEDINGS OF THE ACADEMY OF [1892. 



formation is more complete, p^ — * being altogether like the molars, 

 though, as we have seen, the elements were already present in 

 Hyrachyus. 



In the later genera of the rhinoceros line, such as Aphelops, Bhi- 

 nocerus, etc., the last three premolars in both jaws are hardly to be 

 distinguished from molars. It will be unnecessary to describe the 

 evolution of the premolars in the Trijdopus-Hyraeodon series, as in 

 them essentially the same stages may be noted. 



Turning now to the Titanotheriam line, we find a very similar 

 order of events. The most ancient genus which can be referred to 

 this series is the AVasatch Lamhdothermm, in which P2 ^"^ 3 are tri- 

 gonodont, consisting of proto-, deutero- and tritocones, with a more 

 or less clearly defined anterior conule. One important diflference, 

 however, between the upper premolars and molars should be noted, 

 viz., that in the former the external cusps are convex, more or less 

 strongly, on]the outer side, while the external cusps of the molars 

 are deeply concave on the outer side, and thus form V-shaped 

 crests. In the lower jaw p^ is very simple; it consists of a high, 

 acute protoconid, a well developed metaconid and an anterior basal 

 elevation which may represent a rudimentary paraconid. On p 

 the metaconid is enlarged and a minute deuteroconid makes its 

 appearance upon the internal or lingual side of the protoconid. 

 P* is almost molariform ; the proto- and deuteroconids form a crest 

 and a very low and inconspicuous tetartoconid is added. However, 

 neither the anterior nor the posterior V characteristic of the molars 

 is complete, owing to the rudimentary character of both the para- 

 aud tetartoconids. 



The Bridger genera, Palceosyop-'< and Tehnatherium, aside from their 

 larger size, have premolars very much like those of Lambdotherium, 

 only the fourth inferior premolar has the V-shaped crests better 

 developed through the elevation of the para- and tetartoconids. In 

 Telmatherium this elevation has proceeded so far that the tooth may 

 fairly be called molariform. In the upper jaw we observe the same 

 striking diflTerence between the convex external lobes of the pre- 

 molars and the deeply concave ones of the molars. Another 

 change is the reduction or suppression of the conules in both classes 

 of teeth. 



This series is represented in the Uinta beds by Diplacodon, in 

 which Pi has become like the molars through the addition of the 



