202 ON THE EXTINCT MAMMALIA OF 



ANTIIRA GO TIIERID2E. 



This extinct family, whose types are the genera Anthracothermm and Chocropota- 

 mus of the early and medial tertiary formations of Europe, is represented in the 

 tertiam/^ deposits of the Mauvaises Terres by a species oi Eyopotamus, also a European 

 genus. Its remains were derived from the lowest stratum of the miocene formation, 

 marked as bed A by Dr. Hayden, especially distinguished by the abundance of 

 remains of a huge associate, hereafter to be described under the name of Tltano- 

 tlierium. 



HYOPOTAMUS. 



The genus Hyopotamus* was established by Prof. Owen on some remains found in 

 the eocene formation of the Isle of Wight, though it appears to have been previously 

 noticed by M. Pomel under the name of Ancochos, from some remains found in a 

 deposit of the same age in France. 



The dental formula of the animal was probably the same as in the Hog and the 

 extinct Anthracotherium ; that is to say, it possessed three incisors, a canine, four 

 premolars and three true molars on each side of both jaws. 



Htopotamus americanus. 



Among the most interesting fossils obtained by Dr. Hayden in the Mauvaises 

 Terres of White River are the remains of a species of Hyopotamus, to which the 

 above name has been given. They belong to a single individual of adult age, and 

 consist of upper molar teeth with small portions of the jaw attached to them, and 

 part of a lower molar. 



The upper teeth are the true molars of the left side, the anterior two of the oppo- 

 site side, and what I take to be, in comparison with the teeth of Anthracotlierium 

 magnum, the second premolar of the left side and the second and third of the right 

 side. A series of the teeth is represented in figures 1, 3, plate XXI. The portion 

 of a lower molar belongs to the last of the series, and consists of the crown without 

 its hinder lobe, represented in figures 5, 6, plate XXI. 



The teeth of Hyopotamus americanus are about the same size as those of H. bovmus 

 of the Eurojiean tertiary formation, and the corresponding ones are so much alike, 

 that Prof. Owen's description of the upper true molars of the latter applies nearly 

 equally well to those of the former. 



The crowns of the upper true molars, figure 1, are quadrate, with the transverse 



*In furnishiag the list of ostinot mammals of Dakota and Nebraska for the table of Dr. Haydeu, pages 20, 

 21, I iuadvertently placed this geuus and the next among the Perissodactyla. 



