THE MAMMALIA OF THE UINTA KOHMATION. 



533 



a trochlear character, the liljiila resting upon the calcanenm throughout extension, as 

 in Macrauchenia and the ArtiodactyUu (2) In the primitive Equidto and in the Titano- 

 therida-, the fibula rests upon the calcanenm in extreme tlexion. In ancestors of 

 the latter family we observe this facet developing evidently as an adaptation to groat 

 body weight. (3) The third type of libulo-calcaneal facet, as seen in the Proboscidia, 

 (?) Dinocerata, and Toxodon,* in which the ankle motion is limited and the (ibula 

 rests broadly upon the calcanenm, is also probably a secondary adaptation to weight. 

 As in several instances this facet has been acquired secondarily, as it is wanting in the 

 majority of primitive taxeopods, and is not essential to the perfect working of the 

 plantigrade ankle joint — the evidence as to its primitive character is certainly not 

 conclusive.t 



A B. 



DiAOS. fi-7. — Tliiid linil) of Periptychus rliahdodon Copo, from original in the Cope collection. A, Front view of 

 pes, X \. D, Siili- view orriL'lit himl linil). X I- Tlir rc.-torfMl and conjectural portions are in dolicil lini-j. <if, Aslmg- 

 alar foramen. 



The trochlear groove of the astragalus faced upwards, when the foot was 

 prone, and with its slight concavity and obtuse edges allowed considerable lateral 

 movement. The antcro-posterior movement was evidently very limited, for this facet 

 in all the Pucrco genera is small ; it was indented by a foramen probably transmitting 

 the flexor <l>f/Horiim cmnmunis. The importance of this astragal a r foramen as 



• " In Taxodon tlie fibula articulation is of unusual size." Cope, Proc. Am. Phil. Soc, .\|>ril, I8.SI, p. 'lOS. 

 f Wc are spcakins; of primitive unf;ulales, not of primitive Mammalia in general. 



A. P. S. — VOL. XVI. 3p. 



