ON THE ORIGIN OF GENERA. 85 



acters more nearly resembles the Amiiwus lynx, than the latter 

 does the A. alhidus and many other species of the genus. The 

 A. hjnx is found in the same streams. The important generic 

 cliaracter, the absence of eyes, is, however, its constant feature 

 (in three specimens known to naturalists, others to fishermen). 



5. The Cinclidium maximum, a large tree-toad of Brazil, re- 

 sembles in all its characters the Centrotehna geograpliicum. The 

 sjiecific differences between them amount to almost nothing, but 

 both sexes of the former grow larger and are furnished with a 

 generic peculiarity in tlie addition of some phalanges to the 

 thumb. 



6. The Oporornis agilis, Baird, a North American bird of the 

 Tanager family, resembles very closely, in form, color, and habits, 

 the adjacent species of the adjacent genus Geothlypis. While its 

 specific characters are thus very close to Geothlypis teplirocotis, 

 it differs in the generic feature of a longer wing. By this it is 

 associated, and properly so, with another species 0. formosiis, 

 which has the general color and habits of species of Myiodioctes 

 ( Jf. canadensis), the next related genus. 



7. The following fact I give on the authority of Prof. Leidy, 

 who will publish it in his forthcoming work on the extinct Mam- 

 malia of Nebraska, etc. 



Three species of Oreodon occur in the Miocene strata ; tliey 

 are a larger, a medium, and a small sized species. In the Plio- 

 cene beds above them they are represented by three species of 

 Merychyus, which are in all respects known, identical specific- 

 ally with the three preceding. Each one may thus be said to be 

 more nearly allied to the species of the other genus than to its 

 fellow of the same genus, in specific characters. But each, on the 

 other hand, differs from each in generic characters. The teeth of 

 Merychyus are more prismatic, have longer crowns and shorter 

 roots, approaching the sheep, as Oreodon does the deer.* 



* This phenomenon suggests an explanation on the score of adaptation, which the 

 other cases do not. The existence during the later period of a tougher material of 

 diet would increase the rapidity of wearing of the crown of the tooth, and require 

 a longer crown and greater rapidity of protrusion. This necessitates a dimi- 

 nution of the basal shoulder and shortening of the roots, producing the prismatic 

 form aforesaid. The deer browse on forest foliage, which is more tender, while 

 the Cavicornia graze the grasses, which contain, as is known, a greater amount of 

 silex ; hence the more rapid attrition of the tooth. 



This may have been the case with the two extinct genera ; the different periods 



