354 Bulletin American Museum of Natural History. [Vol. XXYII, 



II. The Condylarthra. 



Phenacodus and Euproiogonia. 



The famous Phenacodus primcevus of the Lower Eocene was described 

 and figured by Cope (1884, pp. 435-463, pll. Ivii b et seq.) and by Osborn 

 (1898, pp. 159-164, pi. xii). It was at first regarded as the "five-toed 

 atavus" of the Perissodactyls and especially as ancestral to H ijracotherium 

 and the Equidc-e; but Matthew (1897, pp. 309-310) and Osborn (1898, pp. 

 163-164) showed that Phenacodus and even its more generalized Basal 

 Eocene ancestor Euproiogonia were only indirectly related to the ancestors 

 of the Perissodactyla and w^ere in fact a cursorial hoofed offshoot of a com- 

 mon Condylarth-Creodont stock. 



The derivation of Phenacodus and Euproiogonia from the ancestors of the 

 Insectivore-Creodont group is indicated by many primitive characters, as 

 follows : 



(1) The dental formula gxTT' ^^ especially characteristic of the 

 Insectivore-Creodont stock and its descendants. 



(2) The upper molars are derived from the tritubercular type by the 

 addition of the hypocone, the lower by the reduction of the paraconid. In 

 Euproiogonia the derivation of the molars (figured by Osborn, 1907, p. 169) 

 from a tritubercular type analogous to that of the supposed Insectivore 

 Hyopsodus laiicuneus (Osborn, /. c, p. 149) is very obvious. In the Basal 

 Eocene genus Proiogonodon Scott, the hypocone is only incipient and the 

 molars are tritubercular (see Osborn, /. c, p. 169, fig. 148). 



(3) The skull in Phenacodus and Euproiogonia resembles the primitive 

 Creodont type in its heavy muzzle, small brain case, long low sagittal crest 

 and rather slender zygomata. An alisphenoid canal, into which, as in the 

 dog, the foramen rotundum oj^ened, was present. Derivation from Insec- 

 tivorous ancestors may also be indicated by the rather short erect canines, 

 elongate premaxillaries and slightly procumbent lower incisors (P. tvorimani). 



(4) The brain (Cope, 1884, pi. Ivii b) was of a very low type, with large 

 olfactory lobes, and small smooth cerebra which probably left the corpora 

 quadrigemina and cerebellum uncovered. 



(5) The skeleton, which has been figured by Cope (1884) and Osborn 

 (1898) also retains many archaic Insectivore-Creodont features, such as the 

 long tail, arched back, hatchet-shaped spine of axis, entepicondylar foramen, 

 large shaft of the ulna, third trochanter on the femur, pentadactyl manus 

 and pes, etc. There were 20-21 dorso-lumbar vertebrae (Osborn), i. e., 

 very near the characteristic Creodont number of 20. The "serial" arrange- 



