1892.] NATURAL SCIENCES OF PHILADELPHIA. 441 



Thus, the genesis of the third upper milk molar (dj) in the Oreo- 

 dontidcE. shows that in that tooth, at least, the order of succession of 

 the cusps is different from that which occurs in the premolars, but 

 on the other hand, the homologies of these cusps, as determined by 

 their position, is the same as in the premolars. In Oreodon ^3 con- 

 sists of three cusps, of which the anterior one is extended and tren- 

 chant and obviously corresponds to the protocone of the premolars ; 

 I^osterior to this are two smaller crescentic cusps, the outer one of 

 which is plainly the tritocone and the inner one the tetartocone. In 

 Merychyus this tooth is rendered molariform by the addition of the 

 antero-internal crescent, or deuterocone. The order of succession 

 of the cusps thus appears to be: proto-, trito-, tetarto- and deutero- 

 cones, and is, therefore, different from that which is typical of the 

 premolars, though, as we have already seen, there are variations in 

 the order of development in the different teeth of the premolar 

 series. For example, in Hyracothermm P^ consists of the proto- 

 and tritocones only. 



Some light is thrown upon the homologies of the elements of the 

 ■characteristic six-lobed last lower milk molar of the artiodactyls 

 by the curious and problematical little genus Nanohyus. Not that 

 this genus is of any phylogenetic significance for the artiodactyls, 

 and indeed its systematic position is quite uncertain ; nevertheless, 

 it exhibits d* in a very interesting and instructive stage. This tooth 

 is thus described by Leidy : " Its crown presents the usual greater 

 breadth than the succeeding pair of those of the teeth behind as in 

 pachyderms. The crown is trilobate, externally and internally, and 

 this condition probably corresponds witli three constituent pairs of 

 lobes, the distinction of which is, for the most part, obliterated by 

 wearing. The median division of the crown is largest and that in 

 advance is the smallest. The abraded summit of the former pre- 

 sents an irregularly transverse quadrate surface of exposed dentine, 

 continuous with a smaller subreniform tract upon the anterior 

 division. The posterior division of the crown still exhibits the dis- 

 tinction of a transverse pair of lobes, of which the outer one is 

 much the larger." In this tooth the homologies of the elements 

 are obvious ; the median pair of cusps are the proto- and deutero- 

 conids respectively, and the posterior pair the meta- and tetartoconids, 

 while the paraconid and a cusp, as yet not named, internal to it, 

 form the anterior pair. In Nanohyus, therefore, d* differs from the 

 typical artiodactyl tooth merely in the relative development of the 



