124 THE MAMMALIA OF THE DEEP RIVER BEDS. 



much more closely approximated and sometimes confluent. (11) In the humerus, 

 the deltoid crest is much better developed. (12) In the carpus, the magnum does 

 not support the lunar anteriorly, and the latter element has shifted more completely 

 upon the unciform. (13) The fifth digit is reduced to a nodular rudiment of the 

 metacarpal. (14) "The femur of the species from the earlier formations may be 

 readily distinguished from that of those of the later Tertiaries by the forms of both 

 the extremities. In the Aceratheria, this bone resembles that of the tapirs in the 

 form of the great trochanter. This process is produced at its external border, has a 

 recurved apex, and encloses a deep trochanteric fossa. In Aphelops it is precisely as 

 in Uliinocerus, obliquely truncate externally, without prominent apex or well-marked 

 fossa. In the Aceratheria the inner crest of the rotular groove is but moderately 

 prominent; in Aplielops and Rhinocerus it is greatly developed" (Cope, No. 4, 

 p. 771 e ). (15) The astragalus has become lower and broader and has a much more 

 extensive bearing upon the cuboid, and the calcaneum is shorter and more massive. 



With these resemblances, Aplielops presents many divergences from the true 

 rhinoceros series, which Osborn has thus summed up: "The subtriangular shape of 

 the scapula, the very elevated position and sessile character of the deltoid ridge of 

 the humerus, the spreading manus, and the comparatively feeble development of the 

 third trochanter of the femur" (No. 28, p. 98). To these may be added certain con- 

 stant differences in the character of the skull. The presence of horns in one series 

 and the absence of them in the other is doubtless the cause of these divergences in 

 skull structure. Leaving out of view the problematical Diceratherium — a genus 

 which is common to both hemispheres, and the relationship of which to the other 

 genera of the family is still far from clear — all the American forms have weak and 

 slender nasals ; the sagittal crest is retained, in striking contrast to the broad, flat- 

 tened cranium of the horned genera, and the development of air sinuses in the bones 

 which surround the cerebral cavity is carried only to a moderate extent. Still farther 

 differences between the two series occur in the details of the tarsus and the mode of 

 articulation of these bones with the metatarsus. 



It may be fairly concluded that the American hornless genera, while running 

 parallel to the horned rhinoceroses of the Old World in many very striking ways, 

 nevertheless form a series entirely independent of them. 



