1910.] The Perissodactyla. 385 



IV. Order Pyrotheria. Molars bilophodoiit, tapiroitl to Dinotheroid. 

 Limbs rectigrade. Astragalus greatly flattened. 



1. Molars tapiroid. CarolozitteUa. 



2. " Dinotheroid. Pijrotherium, Propijrotherium. 



IX. The Perissodactyla. 



The question of the derivation of the Perissodactyls as an order and of 

 their relationshij) to the Condylarthra is still open. Cope saw in Phena- 

 codus the atavus of practically all the hoofed orders. Osborn holds the 

 contrary opinion that Phenacodus is a hoofed offshoot of the Creodonta and 

 a member of the Meseutheria, or small brained Cretaceous-Basal Eocene 

 orders, and that the Perissodactyls have sprung from some entirely unknown 

 "Cfeneutheria" (p. 457). 



Still another view may be adduced: that although neither Phenacodus 

 nor Euprotogonia were the ancestors of the Perissodactyla yet they resemble 

 those forms more nearly than do any other known mammals; and that the 

 Basal Eocene ancestors of the Perissodactyla would, if discovered fall under 

 the superorder Protungulata as defined above (p. 383). 



Supposed relations tvith the Artiodactyla. 



The frequently assumed relationship of the Perissodactyla with the 

 Artiodactyla is treated in a general way in the section on the Artiodactyls 

 (p. 400) but it seems desirable to touch here upon a particular phase of the 

 question, namely, the supposed Artiodactyl characters of the Oligocene 

 Titanotheres. These characters were noted by Osborn (1893, p. 94 and 1895, 

 p. 350) as follows: (1) the dorso-lumbar formula of 20 (Osborn); (2) the 

 four-toed manus, which superficially resembles that of Hippopotamus; 

 (3) the buno-selenodont molars which, especially in some genera of the 

 Palseosyopidse resemble those of the Artiodactyl Anoplotherium. To these 

 may be added: (4) the large size of the cuboidal facet of the astragalus; 

 and (5) the reduction of the third trochanter of the femur. 



Now that the general phylogeny of the Titanotheres has been cleared up 

 by Osborn in various publications (1896, 1902, 1908) it is plain that the 

 characters given above, with the exception of the first, have all been acquired 

 within the Titanothere phylum and therefore constitute no evidence of close 

 relationship with the Artiodactyls. 



(1) The 20 dorso-lumbars may be simply a primitive Placental char- 

 acter which only the Titanotheres among the Perissodactyls have retained 



