98 BULLETIN OF THE 



Comparison with Aceratherium and Rhinocerus. 



There is nothing, however, which precludes the supposition that the Ameri- 

 can lower and upper Miocene Aceratheria are genetically related. 



All portions of the skeleton of A. occidentale are now known to us, excepting 

 the scapula, pelvis, and dorso-lunibar vertebrae; they indicate an animal in the 

 same stage of skeletal evolution as the recent tapir; the proportions are practi- 

 cally similar; the displacement of the carpals and tarsals is in a corresponding 

 stage. The mode of progression was also probably similar, for all the articular 

 facets and protuberances for muscular attachment present innumerable points 

 of resemblance. Cope 2 first pointed out the tapir resemblances in Aceratherium, 

 especially in the separation of the foramina spheno-orbitale and rotundum ovale 

 and foramen lacerum medium; the separation of the post-glenoid and post- 

 tympanic; and the form of the femur. We have shown that this resemblance 

 applies to the carpus 2 and tarsus; it is also true of the humerus and forearm, 

 and of the atlas and axis. The remaining cervicals are widely different ; it is 

 probable, also, that the pelvis and scapula were different. This is of course 

 simply an instance of functional and structural parallelism. It follows that an 

 enumeration of the differences between the recent tapir and rhinoceros would 

 also embrace the majority of the features which distinguish Aceratherium from 

 Aphelops, for the latter is in most respects a fully developed rhinoceros. 



Thus, if the descent from Aceratherium to Aphelops took place, it was accom- 

 panied by wide-spread modifications of the skeleton. In Aphelops megalodus we 

 find a probable transition species. Its proportions are more intermediate. The 

 narrow elevated occiput, the less degree of separation of the foramina of the 

 skull, the lophodont character of the first upper premolar, the small develop- 

 ment of the * anticrochet ' in the superior molars, — these characters all point 

 towards Aceratherium. 



A.fossiger is a highly modified form, with its broad occiput, simple first pre- 

 molar, and confluent cranial foramina. In many respects the modifications it 

 exhibits are simply steps towards the recent rhinoceros type; for example, its 

 tridactylism, the extreme displacement of the podials, and the characters of the 

 spinal column. But there are many points in which Aphelops differs from the 

 recent rhinoceroses; namely, the sub-triangular shape of the scapula, the very 

 elevated position and sessile character of the deltoid ridge of the humerus, the 

 spreading manus, the oval obturator foramen, and the comparatively feeble de- 

 velopment of the third trochanter. The marked peculiarity of the upper molars 

 is the development of both the ' crochet ' and 'anticrochet.' and absence of the 

 ' crista.' This combination is very distinctive, since all the living rhinoceroses 

 present combinations of the 'anticrochet' and 'crista.' 8 The molars of Aphelops 



i Bull. U. S. Geol. Surv., Vol. V. No. 2, p. 235. Also, "On Extinct American 

 Rhinoceroses and their Allies," Am. Nat., Dec, 1879, p. 771 c. 



2 Osborn, "Evolution of the Ungulate Eoot," Mem. Uinta Mamm., p. 550. 



3 See Flower, " On some Craniai and Dental Characters of the Existing Species 

 of Rhinoceroses," Proc. Zool. Soc, 1876. 



