THE MAMMALIA OF THE DEEP KIVER BEDS. 123 



characteristic parts of the skull, the specific reference of this specimen is uncertain. 

 The antero-posterior concavity of the upper profile of the cranium, and the rise 

 towards the occipital crest, constitute a resemblance to A. megalodus, though these 

 features are less emphasized than in that species. On the other hand, the projecting 

 occipital condyles and long, laterally compressed postglenoid process, with its rugose 

 posterior border, are rather more like those of A. fossiger. The posttympanic pro- 

 cess of the squamosal does not quite reach the postglenoid, while in most specimens 

 from the upper Loup Fork beds there is a more or less extensive contact between the 

 two processes, though they do not seem to be coossified. 



As I pointed out several years ago (No. 35, p. 16), the line of the horned 

 rhinoceroses of the Old World diverged at an early period from the American horn- 

 less series ; the two phyla cannot well have any common ancestor more recent than 

 the Aceratheria of the "White River Oligocene. It follows from this that those fea- 

 tures in which Aphelops and its congeners resemble the modern genera more closely 

 than do the White River species, have been independently acquired in the two lines. 

 These resemblances are numerous and, because of the confidence with which we may 

 regard them as parallelisms, worthy of enumeration. 



The following brief summary of the points in which Aphelops approximates the 

 modern standard more than do the "White River forms is taken principally from the 

 papers of Cope and Osborn upon that genus. (1) The increased size and robustness 

 of the skeleton, as compared with that of the older genus, are very marked, and in 

 some species (e. g., A. fossiger} carried even beyond the condition of the recent spe- 

 cies, so as to produce, as Cope has shown, the proportions of the hippopotamus rather 

 than those of any recent rhinoceros. (2) The upper incisors are reduced to a single 

 one in each premaxillary. In the Loup Fork genus, Peraceras, these teeth have been 

 lost entirely, as in the recent African form, Aielodus. (3) The superior premolars 

 have become more thoroughly molariform by the more complete sepai'ation of their 

 internal cusps (deutero- and tetartocones). (4) The upper molars have increased in 

 size and have become more complicated through the development of spurs upon the 

 transverse crests. (5) The shape of the occiput is much more modernized than in 

 Ccenojjus, and in most of the species the upper contour of the skull is concave, rising 

 more or less steeply towards the inion. (6) The postglenoid and posttympanic pro- 

 cesses of the squamosal are in contact. (7) The postglenoid is less like that of the 

 tapir in character and has an elongated styliform shape, as in the recent rhinoceroses. 

 (8) The sagittal crest is shorter and less prominent and the cranial cavity more 

 rounded and capacious. (9) The foramen lacerum anterius and foramen rotundum 

 have become confluent. (10) The foramen lacerum medium and foramen ovale are 



