202 CONDYLARTHRA 



here collected it is clear that a direct effect is produced. If we 

 are to regard horns as secondary sexual appendages which have 

 been subsequently handed on to the female by heredity, we should 

 expect to meet with examples of animals now horned in both 

 sexes, of which the earlier representatives had the horns confined 

 to one sex. This is most interestingly shown by the extinct and 

 Miocene Giraffe, Samotherium, of which the male alone had a pair 

 of short horns, while the skull of the female was entirely hornless ; 

 the modern Giraffa, as is well known, has horns in both sexes. 



It is interesting to note that the existing Perissodactyles and 

 Artiodactyles are to be distinguished by their unpaired or paired 

 horns. But while there are.no Artiodactyles with unpaired horns 

 (save occasional sports) the Perissodactyles have more than once 

 tried, so to speak, paired horns, which ultimately proved fatal 

 to them. The Rhinoceros Diceratherium apparently inherited and 

 improved upon the small paired horns of Aceratherium, but it has 

 left no descendant. The paired horned Titanotheria offer another 

 instance of the same apparent incompatibility between the Perisso- 

 dactyle structure and the persistence of paired horns. 



SUB-ORDER 1. CONDYLARTHPvA. 



This group is characterised by the following assemblage of 

 characters. Extinct, often plantigrade Ungulates, with five-toed 

 limbs. Bones of carpus and tarsus not always interlocking, but 

 sometimes lying above each other in corresponding positions. 

 The humerus has an entepicondylar foramen. Dental formula 

 quite complete ; the molars brachyodont and bunodont. The 

 premolars are simpler than the molars. The canines are small. 

 As with other early types, the zygapophyses are flat and do not 

 interlock. The astragalus is like that of the Creodonta. This 

 group was American and European in range, the remains of its 

 rather numerous genera being of Eocene time. The best-known 

 genus is Phenacodus, of which some account will be given before 

 discussing the, in many cases, more fragmentary remains of 

 other allied forms. 



The genus Phenacodus was first described so long ago as 1872, 

 from a few scattered teeth. Since then several nearly complete 

 skeletons have been obtained, and we are in full possession of 



