NO. 2 A STUDY OF MENISCOTHERIUM GAZIN 85 



lacking the acromion and metacromion, so well developed in Menis- 

 cotherium. In the pelvic girdle the ilium is relatively very long and 

 straight, quite unlike that in Phenacodus or Meniscotherium. 



In the forelimb of Procavia the proximal extremity of the humerus 

 is similar to that in Meniscotherium, but the distal trochlea shows a 

 more simple, grooved pulley construction and there is no entepi- 

 condylar foramen. The distal articular surface of the ulna for the 

 cuneiform is highly concave rather than convex as in Meniscotherium, 

 and that of the radius for the scaphoid and lunar is distally extended 

 on the scaphoid side, binding the carpus medially, much as the 

 peculiar extension of the medial side of the tibia effectively binds the 

 astragalus medially. Individually the elements of the carpus bear 

 almost no resemblance in detail to those of the Eocene condylarth, 

 although there is some approximation to the serial alignment. The 

 recent hyracoid carpus, moreover, retains a separate central, and the 

 first digit is reduced to a vestige. 



Differences in the hind limb are noted in the very proximal position 

 of the third trochanter and the very broad patellar trochlea on the 

 femur and the tendency toward coossification of the tibia and fibula, 

 as well as of the radius and ulna in the fore limb. The tarsus is notice- 

 ably different in the peculiar offset of the neck and head of the 

 astragalus, and the articulation between the astragalus and calcaneum 

 is principally through the enlarged ectal facet, there being no sustentac- 

 ulum on the calcaneum. Moreover, the articulation between the 

 calcaneum and cuboid is nearly flat, as is that between the astragalus 

 and navicular. 



Pleuraspidotherium from the Paleocene at Cernay, France, would 

 appear to be condylarthran and possibly related to Meniscotherium. 

 Moreover, there seems to be logic in including them in the same 

 family, although some of the differences are rather striking, so that 

 I cannot believe that Meniscotherium was derived from this form. 

 Possibly P 3 -M 3 in Meniscotherium could have evolved from teeth of 

 the form seen in Pleuraspidotherium, but the long diastema ahead of 

 these teeth in the latter, the reduction in size and single-rooted char- 

 acter of the anterior upper premolars, as well as the frequent reduc- 

 tion in number of the lower series (see D. E. Russell, 1964), would 

 surely preclude it as an ancestor. 



The problem of relationship may be further complicated by a 

 question of homology, relating to certain upper molar cusps. Simp- 

 son (1929) in discussing Pleuraspidotherium and Othaspidotherium 

 in relation to Meniscotherium refers to the posterointernal cusp as a 



