364 SCOTT. [Vol. V. 



of vertebrae and ribs, may be similarly brought about in very 

 different groups. 



When we come to examine positive differentiation, the same 

 parallelism is abundantly proven. The prismatic, cement- 

 covered molar, has -been independently developed in many 

 forms ; e.g. several of the ruminants, certain pigs, the horses, 

 one of the rhinoceroses {Elasmotheriuvi), the elephants, many 

 rodents, etc. The resemblance of the molar of Elasmotherhivi 

 to that of the horse is very striking, but its fundamental plan is 

 rhinocerotic and not equine. The scalpriform incisor, growing 

 from a persistent pulp, is repeated in many very different orders 

 of mammals. Within narrower limits, the selenodont molar- 

 pattern has been several times independently evolved : 1) in the 

 true ruminants ; 2) in the camels ; 3) in the oreodonts, not to 

 mention the somewhat aberrant types of dentition exhibited by 

 Anoplotherium, XipJiodon, Cainotherium, and the like. The 

 rhinoceros form of molar has been independently acquired at 

 least twice ; namely, in the rhinoceros and hyracodont lines. In 

 a great many different series of perissodactyls, as well as in 

 Hyrax, the premolars have become molariform ; the dentition of 

 certain creodonts has become extremely cat-like, and that in 

 forms which can have no direct connection with the true cats. 

 There is a wonderful similarity in the mode of development of 

 cerebral convolutions in many different lines of ungulates, a 

 similarity which the fossil series show to have been independ- 

 ently acquired, and which are plainly the necessary outcome of 

 mechanical principles. In the same way we find that the milk 

 dentition of all the primitive selenodonts agrees in character, as 

 does that of all existing members of the group, except the 

 tragulines, which still retain the ancient structure, but the 

 history of the various ruminant lines proves that the modern 

 type has been independently acquired at least three times, in 

 the Pecora, the Tylopoda, and the later Oreodontidtz, such as 

 Merychyns (No. 51, p. 370). The spout-shaped odontoid process 

 of the axis has arisen in the true ruminants, the horses, the 

 camels, and, to a certain degree, in the later oreodonts, such as 

 MerycJiyus. The proximal end of the humerus in the modern 

 Tylopoda is extremely like that of the horse, and the lower 

 Miocene representatives of these two series, Pcebrotherium and 

 MesoJiippus, also agree almost exactly as to the structure of the 



