1892.] NATURAL SCIENCES OF PHILADELPHIA. 437 



The main facts of premolar evolution among the Pecora will be 

 sufficiently explained by a consideration of the series Dicholnme, 

 Gelocus, Prodremotherium and Palceomeryx, without taking into 

 account the numerous small variations which occur amonff the 

 recent forms. Dichohune displays no important advance over Pan- 

 toledes in the structure of the upper premolars, but the last lower 

 one (p^) has, in addition to the proto-, para- and metaconids a dis- 

 tinct tubercular deuteroconid. Gelocus, although a typical rumi- 

 nant and much nearer to the modern types than Dichobune in 

 almost every particular of structure, has, nevertheless, made no 

 noteworthy advance over that genus as regards the character of the 

 premolars. In Prodremotherium P3 has completed the internal 

 crescent, but it is important to notice that the deuterocone is eccen- 

 tric in position and much nearer to the posterior margin of the 

 <;rown than the anterior. P2 has a rudimentary deuterocone. In 

 the lower jaw p* has, when seen from the outer side, a trilobate 

 appearance, the proto-, para- and metaconids being all present ; in 

 addition to these there is a well marked deuteroconid and a less 

 distinct tetartoconid which are connected by very short transverse 

 ridges with the external cusps which stand opposite to them. On 

 p-iand^ these internal elements are indicated only by faintly marked 

 ridges. Pakeomeryx has all the upper premolars of the same pattern, 

 the inner crescent being now completed on the most anterior tooth (P2) 

 and the deuterocone having assumed a symmetrical position on £3. 

 The inferior premolars are like those of Prodremotherium, except 

 that on pSaiid^ the internal cusps and transverse ridges are much 

 better developed and the enclosed valleys deeper ; p- is somewhat 

 reduced. From this condition that of the lower teeth in the vari- 

 ous recent families may readily be traced. 



It would not be worth while to describe the numberless minor varia- 

 tions of the premolars which occur in the various genera of extinct 

 selenodonts contained in the families of the anoplotheres, anthra- 

 cotheres, xiphodonts, etc. It will suffice to mention some of the 

 more salient peculiarities. In the problematical little Hypi^odus of 

 the White River, the inferior incisors, canines and first two premo- 

 lars form a continuous series of twelve subequal teeth. Xiphodonthe- 

 rium has lost the first lower premolar and p^ has become caniniform. 



In Rhagatherium P4 is molariform and has five cusps, the ante- 

 rior conule being present in addition to the four main elements. 

 P* is exceptional in being like its predecessor in the milk dentition ; 



