80 THE MAMMALIA OF THE DEEP RIVER BEDS. 



Miohippcts aistntectens ? Marsh. 



(PI. I, Figs. 6-8.) 

 (loc. cit.) 



A fine specimen of a hind limb from the lower beds, not accompanied by teeth, 

 corresponds closely to the measurements given by Marsh of this, the type species of 

 the genus. The femur differs little, except in size, from that of Mesoliippus ; it is 

 relatively longer, more nearly equalling the length of the tibia. The head rises as 

 much above the bridge which extends to the great trochanter, but is more approxi- 

 mated to that process, and the pit for the round ligament is wider and more equine 

 in shape. The head projects to a remarkable extent in front of the anterior plane of 

 the shaft. The shaft is laterally compressed, and narrow when seen from the front, 

 but of considerable antero-posterior diameter. The pit above the external condyle 

 for the attachment of the plantaris muscle is very deep and conspicuous. The con- 

 dyles are rather small and do not project strongly backward, but are more prominent 

 and separated by a deeper groove than in MesoMppus. Another difference from the 

 latter genus is seen in the greater elevation and thickening of the inner border of the 

 rotular trochlea, though this by no means attains the proportions found in Equus. 



The tibia is almost an enlarged copy of that of the White Eiver genus. The 

 condyles for the femur are brought somewhat closer together and extended farther 

 backward and the cnemial crest is rather more prominent; the shaft is rather stouter 

 and the distal end more expanded. The inner malleolus is more massive and the 

 grooves for the astragalus somewhat more deeply incised; the inner groove exceeds 

 the outer in antero-posterior extent in a more marked degree. About two inches of 

 the distal end of the fibula is firmly coossified with the tibia, forming a stout external 

 malleolus, which bears a small facet for the calcaneum. The proximal portion does 

 not appear to have coalesced with the tibia, and is lost; nor can we determine 

 whether the exceedingly slender shaft was interrupted. 



Beyond the mere increase in size, the tarsus shows surprisingly little advance 

 over that of MesoJiippus. The calcaneum retains the long, slender tuber with nearly 

 parallel dorsal and plantar borders; the distal part is a very little shorter in propor- 

 tion, so little that the difference is probably individual. The cuboidal facet is longer, 

 more distinctly separated into anterior and posterior portions, and the latter is more 

 incurved toward the sustentaculum, which constitutes an advance toward the mod- 

 ern standard. The other facets show no change. 



The astragalus likewise presents no noteworthy differences other than the exten- 

 sion of the distal end, especially towards the tibial side. The small sulcus which is 

 very generally, though not invariably, found on the navicular surface in MesoMppus, 



