86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



"pseudhypocone" or displaced metaconule. This would imply a cusp 

 origin similar to that of the early selenodont artiodactyls, and the up- 

 per molars do show a surprising resemblance to some of the seleno- 

 dont forms of the later American Eocene. Support of the metaconule 

 origin for the posterointernal cusp might be inferred from the devel- 

 opment, although weak, of the cingulum around the "pseudhypocone" 

 in Pseudaspidotherium. The hypocone from an origin on the cingu- 

 lum would less likely show this, unless a secondary cingulum were 

 developed. The evidence from M enisc otherium would appear to 

 contradict such an origin for the posterointernal cusp, as this cusp 

 seems to have originated from the cingulum, from the evidence of the 

 premolars, for what it is worth, as well as from the cingulum itself. 

 P 8 and P 4 of Menisc otherium suggest between them something of 

 the transition in the relations of the generally well denned accessory 

 cuspules to the deuterocone and tetartocone. Moreover, the pos- 

 terointernal cusp of the molars, as well as the tetartocone of the pre- 

 molars, seems to be an integral part of the cingulum. The latter 

 shows no tendency to divide as in Pleuraspidotherium. The pos- 

 terior upper premolars are rather Phenacodus-like, as observed by 

 D. E. Russell (1964), so that the relations there of the metaconule 

 and tetartocone are missing. I am much inclined to consider 

 that the Meniscothcrium upper molars, though decidedly selenodont, 

 have a cusp pattern essentially homologous with that in the Tetra- 

 chiowdon-Phenacodus line, with the posterointernal cusp as the 

 hypocone. If it can be demonstrated that the cusp in this position 

 is not homologous in Pleuraspidotherium (and Orthaspidotherium) 

 the meniscotheriid relationship certainly becomes much weakened. 



Russell (1964) has given a detailed description of the skull of 

 Pleuraspidotherium, and Pearson (1927), as well as Russell, has 

 presented the characters of the basicranium. In a comparison of the 

 basicranium of Pleuraspidotherium with that of Menisc otherium, 

 however, a number of features are noteworthy. The pterygoid fossa 

 is well developed, and although the walls are broken down they are 

 not so flaring, and it would appear that the fossa is not carried so low 

 with respect to the basisphenoid or the roof of the narial passage as in 

 Mcniscotherium. Posterior to this fossa there is no evidence for an 

 alisphenoid canal in the position that it is found in M eniscotherinm. 

 The foramen ovale is prominently displayed in about the same posi- 

 tion relative to the foramen lacerum medium, but less widely removed 

 from the glenoid surface. The glenoid surface, though not complete 

 in the specimen examined, does not appear to be so elongate antero- 



