322 MAMMALIA. 



but still struggling for existence where competition happens to 

 be least severe in their particular case. 



Some of the earliest Eocene perissodactyls are so remark- 

 ably similar to the tapirs, that they may be claimed with much 

 probability as the direct ancestors of the latter. Systemodon, 

 for example, known by the skull and dentition, perhaps also by 

 part of the hind limb, from the Wasatch Formation (Lower 

 Eocene) of Wyoming and New Mexico, seems to be one of 

 these. It is a small animal with all the premolars simpler 

 than the molars; and its hind foot probably bore a rudiment 

 of digit V in addition to the usual nos. II, in, iv. Otherwise 

 it is essentially a tapir. Contemporaneous allied forms in the 

 same stage of evolution, so far as teeth and feet are concerned, 

 incline less towards the tapirs but more distinctly in the 

 direction either of the rhinoceroses or of the horses. Hence 

 the perissodactyls at the base of the sub-ordinal series may be 

 classified provisionally in one or more families comprising only 

 generalized types; or they may be distributed for the most 

 part in the surviving families of Tapiridse, Rhinocerotidse, and 

 Equidse, according as they seem to be ancestral to the one or 

 to the other. The first-mentioned arrangement seems prefer- 

 able at present, since it is not based on any hypotheses as to 

 the genetic succession of certain forms ; but it must be under- 

 stood that on neither plan can the families be precisely 

 diagnosed. 



The most generalized of the early perissodactyls are thus 

 grouped in the family of Lophiodontidae, so named from the 

 typical genus Lophiodon, which is known by the skull, dentition, 

 and a few other bones from the Lower and Middle Eocene of 

 Europe. The dentition forms a complete series, there being 44 

 teeth in all, and the grinding teeth are brachyodont. The 

 tubercles on the molars of each jaw tend to fuse into two 

 transverse ridges (i.e., become bilophodont), but the premolars 

 are smaller and simpler. So far as known, the digits number 

 four in the manus and three in the pes, precisely as in the 

 Tapiridse. In some genera the two ridges on the molars are 

 more or less separate throughout ; among these are included 

 Systemodon, already mentioned as probably an ancestral tapir, 



