1889.1 ^•^■*- [Cope. 



cerned. In Protohippua, on the other hand, the junction has already taken 

 pbice, but as the columns are of equal size, the pattern resulting is differ- 

 ent from that of Equus. It is that of HippiJium, which, like Equus, 

 has but one digit. An approach to Hippidium is seen in the species of 

 Group II of the analytical table below, to one of which I gave the name 

 of Stylonus. In this type the internal columns are of subequal propor- 

 tions as in that genus. The posterior is joined to the corresponding inter- 

 nal crescent, and though the anterior column approaches very near to the 

 anterior internal crescent b}^ an apex of its section, it never does actually 

 join. On the other hand, the posterior column is distinct in the just-pro- 

 truded molar, and in the H. seversum rcmai)is distinct for a considerable 

 part of the age of the animal. 



The genus Hippodactylus* Cope, has the molar teeth of the general 

 character of Hippotherium, but it possesses, like Hippidium, but one digit. 

 The type is the S. antelopiaum Falconer, of the Sivalik Upper Miocene of 

 India. I think it not unlikely that when the skeletons of some of the 

 species here referred to Hippotherium come to be known, that some of 

 them will be found to belong to Hippodactylus. 



History. — This genus was named by Christol and Kaup within about a 

 year of each other, according to Bronn. I nave not been able to refer to 

 the essay in which the earliest name, that of Christol, was published, but 

 according to Bronn, it was not accompanied by any generic description. 

 In Kaup's first publication an attempt is made to characterize the genus, 

 and although the description contains some errors, the important fact of 

 its possessing dew-claws is mentioned. Two years later Kaup published 

 a fuller and more accurate diagnosis. Christol does not appear to have 

 ever given a description of the genus. The use of his name (Hipparion) 

 does not therefore appear to be warranted. 



Species. — The type is Hippotherium primigenium, the Equus primigenins 

 of Von Meyer and the later i7. gracile of Kaup. Its remains are common 

 in the Mediterranean countries of Europe, and in France. A few other 

 species have been discovered in the Old World. Their liistory has been 

 elucidated by Wagner, Duvernoy, Gervais and others. North America 

 has furnished a larger number of specific forms than any other country. 



The American species of Hippotherium differ in the extent of the pre- 

 orbital facial depression. In H. speciosiim as in H. primigenium, the fosta 

 is confined to the superior part of the facial region ; in H. isonesum it 

 extends downwards to the molar ridge, as in certain species of Proto- 

 hippus. 



The teeth of this genus are most frequently found in a well-preserved 

 condition, and are hence most useful for purposes of determination. The 

 species maj' be ascertained from those of the maxillary bone, though not 

 without some difficulty. This is rather due to differences between the 

 teeth of the same series than to the variability of the characters them- 



* American Naturalist, 1888, p. 449. 



