NO. 2 A STUDY OF MENISCOTHERIUM — GAZIN 8l 



convex and ventrally flat, and there is a small dorsal depression near 

 the anterior margin which itself is not notched or fissured. 



The Phenacodus primaevus phalanges are relatively shorter and 

 broader, and the distal phalanges are more broadly spatulate with 

 little or no posterior neck. The Tetraclaenodon phalanges are long 

 and slender, not flattened, and the distal phalanx in each digit is 

 transversely compressed and very little spatulate, clearly for a more 

 clawlike structure. Matthew (1915) described the Hyopsodus pha- 

 langes as short, with the unguals clawlike, fissured, and not com- 

 pressed. 



SUMMARY OF RELATIONSHIPS 



Although the highly selenodont teeth of Meniscotherium early gave 

 rise to considerable speculation as to its relationships, particularly 

 to such perissodactyls as the chalicotheres and propalaeotheres, to 

 the hyracoids and to the notoungulates, there is no doubt of its 

 condylarthran affinities. While it differs importantly in many char- 

 acters, evidently of more adaptive significance, from the typical con- 

 dylarth Phenacodus, its rather numerous resemblances, presumably of 

 conservative or basic significance, demonstrate a closer affinity to 

 Phenacodus than to any other mammal whose skeleton is adequately 

 known. This relationship, nevertheless, is best expressed by their 

 being retained in separate families of the order. Similarities were also 

 noted in comparisons with the limited skeletal materials of Hyop- 

 sodus available, but while rather striking in certain instances, the 

 degree of relationship is perhaps a little less close than with Phenac- 

 odus. 



Meniscotherium would appear to be rather less like Tetraclaenodon. 

 although the latter is generally regarded as being in an ancestral 

 position with regard to Phenacodus, and it was largely on the basis 

 of this relationship that Matthew (1897) concluded that the more 

 serially arranged foot structure characterizing Phenacodus was not 

 primitive but secondary. The somewhat less fully acquired serial 

 arrangement in Mensicotherium than in Phenacodus must then have 

 been a parallel development, if not truly primitive, as I would not 

 postulate Tetraclaenodon in an ancestral position to Meniscotherium. 



Structural resemblances to the predaceous arctocyonids, with which 

 the condylarths no doubt converged in much earlier time, show cer- 

 tain rather generalized similarities, but the interlocking elements in 

 both the carpus and tarsus and the more significant differences in the 

 astragalus and calcaneum suggest that the relationship to Menisco- 



