386 Bulletin American Museum of Natural History. [Vol. XXVII, 



Moreover in the Oligocene Titanotheres the number of dorsals, 17, is much 

 higher than in typical Artiodactyls (12-14), and in the Upper Eocene 

 Dolichorhimi.s' (Osborn, 1908, p. 612) the number seems to be 15, as in 

 Phenacodus. 



(2) The broad, spreading, four-toed manus is an adaptation to great 

 weight, as it is also in Hippopotamus. The spreading of the digits and of the 

 carpals, especially of the magnum, may be traced with considerable proba- 

 bility within the Titanothere phylum. The detailed characters of the 

 carpals in the side and back view is very different in Titanotheres and 

 Artiodactyls. 



(3) The large size of the cuboid facet of the astragalus is another prog- 

 ressive Titanothere character, and is equally an adaptation to weight, 

 which never attains the development that it does in the Artiodactyls. 



(4) The reduction of the third trochanter is likewise progressive, and 

 it is concomitant with the lengthening of the femur and relative shortening 

 of the tibia and fibula. 



(5) The simple buno-selenodont molars alike in Titanotheres and 

 Artiodactyls result from the reduction and final disappearance of the proto- 

 and metaconules. This character is accompanied by radical differences 

 between the two orders in the premolars, which are often better indicators 

 of ordinal separation than the molars. 



Relatively close relationships of the Lower Eocene Titanotheres to other 



Perissodactijls. 



The general relationship of the Titanotheres to other Perissodactyl phyla 

 and the relatively close interrelationship of all the Perissodactyl families 

 in the Lower Eocene may be made clear in the following remarks on the 

 Lower Eocene (Wind River) genus Lambdotherium Cope, the systematic 

 position of which was long in doul)t. Osborn and Gregory (in Osborn, 1909, 

 p. 599) have shown: (1) that this animal (which was about the size of a 

 coyote), was an early member of the family Titanotheriida", (2) that in many 

 features it is prototypal to the later Titanotheres, with which it is linked 

 structurally through the genera Eotitanops and Mesatirhinus of Osborn. 



As compared with other Eocene Perissodactyls its Titanothere affinities 

 are revealed in the following characters: 



(1) Its skull, judging from the lower jaw, must have been larger and 

 heavier than that of the earliest representatives of the Tapiridte, Equidse 

 or Rhinocerotidje. 



(2) The canines were relatively stout (cf. Palfeosyopinse). 



(3) The upper premolars, especially p'*, are of the Mesatirhinus type 



