1892.] NATURAL SCIENCES OF PHILADELPHIA. 431 



molar crowns, but in their appearance they differ fi'om those teeth, 

 owing to the much greater prominence of the protoconid. 



The existing representative of this line is the tapir which, like 

 the other recent families of the group, has all the premolars, except 

 the first, in both jaws of the molar type. This is brought about 

 merely by the addition of the tetartocone in the upper teeth, and 

 by the elevation of tlie heel in the lower ones. 



In the rhiaoceros series essentially the same steps may be 

 observed. The most ancient known member of the series, though 

 perhaps not in the direct line of descent, is the Wasatch Heptodon, 

 in which the last three upper premolars are composed of the same 

 elements differing only in size and in the degree of separation of 

 the tritocone from the protocone. In all of these teeth, the 

 deuterocone, with its anterior and posterior crests, is present, and in 

 all there is a well marked anterior buttress, formed by the elevation 

 of the cingulum, and which on P4 is as large as in the molars. P^ 

 is extremely small. In the lower jaw p^ appears to be wanting, and 

 the others increase in size and complexity posteriorly. P* has an 

 anterior crest, composed of the proto- and deuteroconids, and a 

 basin-shaped heel, of which the outer and inner cusps are only 

 obscurely indicated. This tooth is, therefore, nearly molariform. 

 In p3 the deuteroconid is present, but not at all prominent, and does 

 not form a crest with the protoconid, and the heel is still smaller 

 and lower than on p*. 



The superior premolars of the Bridger genus Hyrachyus, are 

 essentially the same as those of Heptodon, as are also the inferior 

 series, except that p' has approximated somewhat more closely to the 

 molar condition, owing to the greater elevation of the heel. Hela- 

 letes {Desmatotherkim), however, though contemporary with Hyra- 

 chyus, exhibits an important advance in the appearance of the 

 tetartocone on P3 ^"d ^ as a very minute cusp, hardly separated from 

 the deuterocone. In the Uinta form Amynodon the separation of the 

 deutero- and tetartocones is complete, but the crests, and especially 

 the posterior one, are very low. Even in Aceratheriuvi, the White 

 Kiver rhinoceros, the deutero- and tetartocones are much more 

 closely approximated than are the analogous cusps of the molars 

 (proto- and hypocones), and in advanced stages of wear the pre- 

 molars present a somewhat different appearance from the molars, 

 seeming to have but one internal element, in which both anterior 

 and posterior transverse crests unite. In the lower jaw the trans- 



