•424 University of California Publications in Geology [Vol. 7 



The separation of metaloph and ectoloph as noted in the milk 

 teeth of specimen no. 21056 is a matter of considerable interest 

 in the classification of the Equidae. In Mesohippus the metaloph 

 and ectoloph are separate, and a small transverse ridge or 

 wrinkle may arise from the inner side of the ectoloph in the same 

 situation as that in the milk teeth of the Nevada specimen, no. 

 21056. In Miohippus, as represented by specimens from the John 

 Day series, the metaloph is usually separated from the ectoloph. 

 In Anchitherium, HypoMppus, and Archaeohippus the metaloph 

 is completely united with the ectoloph. The stage of advance 

 of the cheek-teeth in the Nevada specimen, no. 21056, is near 

 that of Miohippus so far as the relation of the metaloph to the 

 ectoloph is concerned, and in this character it differs from the 

 known forms of Anchitherium, Archaeohippus, and HypoMppus. 

 In general form of the cheek-teeth and in the relation of pro- 

 tocone to protoconule, specimen no. 21056 is of the HypoMppus 

 type. In Mesohippus, Miohippus, Archaeohippus, and Anchi- 

 therium, the protocone and protoconule are distinctly separated, 

 in HypoMppus and in the Nevada specimen the protoconule is 

 small, and is almost completely merged with the protocone. 



As the teeth in specimen no. 21056 correspond so closely to 

 the general type of cheek-teeth in HypoMppus, the hypothesis 

 that the milk molars of typical HypoMppus might show the 

 primitive character of the Nevada specimen naturally suggested 

 itself. Leidy's type of HypoMppus affinis, the typical species 

 of that genus, is a milk tooth, but the specimen shows no sug- 

 gestion of separation of metaloph and ectoloph. It is slightly 

 worn, but a corresponding degree of wear in Dm 4 of the Nevada 

 form would not tend in any way to connect metaloph and 

 ectoloph. Dr. W. D. Matthew, who has very kindly examined 

 such milk teeth of HypoMppus as are present in the 'collections 

 of the American Museum of Natural History, finds that in all of 

 the specimens the metaloph and ectoloph are connected. 



The Nevada form represented by specimen 21056 seems, 

 therefore, to represent a type with dentition in general close to 

 that of HypoMppus, but distinguished especially by the less 

 advanced stage of evolution of the metaloph in the temporary 

 molars. 



