888 



D ARCY WENTWORTH THOMPSON ON 



group of Perissodactyle Ungulates, the group which includes the rhinoceros, the 

 tapir, and the horse. 



Let us begin by choosing as our type the skull of Hyrachyus agrarius, Cope, 



FlG. 54. — The pelves oi Archaeopteryx and of Apatornis, with three transitional types interpolated between I hem. 



from the Middle Eocene of North America, as figured by Osborn in his " Monograph 

 of the Extinct Rhinoceroses"* (fig. 55). 



The many other forms of primitive rhinoceros described in the monograph differ 

 from Hyrachyus in various details — in the characters of the teeth, sometimes in the 

 number of the toes, and so forth ; and they also differ very considerably in the 

 general appearance of the skull. But these differences in the conformation of the 



* Mem. Amer. Mus. of Nat. Hist., i, iii, 1898. 



