258 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



iiG. 35. — Falcjcotherlum. 



Fig. 36. — HipposTjus. 



necessary to produce the two Vs of the mandibular dentition of 

 PcdcBotherium, Palceosyops (Fig. 34), Symiorodon, Anchippodus 

 (Fig. 33), and all their allies (Fig. 35, PalcB other ium). 



5. PcdcBotlieriodont type.— Immediately following the form of 

 the Pcdceosyojjs molar we have that of Hipposyus, where inter- 

 mediate tubercles stand be- 

 tween the inner subconic and 

 the exterior longitudinal cres- 

 centoid tubercles. They are 

 compressed so as to be trans- 

 verse, and only need more 

 complete connection with the 

 adjacent tubercles to give the 

 oblique transverse ridges of 

 A ncliitheriiwi, Palceotherium, 

 and Hyracodon, Rlmiocerus, 

 etc. Hipposyus was origin- 

 ally compared with Anchitherium by Dr. Leidy, and the writer in 

 ignorance of his language remarked :* "An interesting annectant 

 form is seen in Lamhdotherium\ procyoiiinum, where the two inter- 

 mediate tubercles which separate the inner cones from the outer 

 Vs in Limnohyus are so developed as to constitute parts of an in- 

 complete pair of transverse 

 ridges wiiich disappear in 

 front of the bases of the 

 outer Vs. They represent 

 the oblique crests of Pcdmo- 

 therium and A^ichitherium, 

 and thus the genus Lamb- 

 dothermm furnishes a sta- 

 tion on the line from Palce- 

 osyops to the horses." Contemporaneously and quite independ- 

 ently Prof. Marsh expressed similar views X as to its affinities. 



A greater longitudinal extent of these ridges or longitudinal 

 expansion of the tubercles in the molars in both jaws, the oblique 

 connections being still retained, gives the type of Equus (Fig. 39). 

 The elevation of the tubercles and deepening of the valleys gives 



* Hayden's "Geol. Surv. Montana," etc., 1873, p. 647. 



f This species was in the original erroneously called Orohippus procyonirvus, 

 (Ed. 1886.) 



X "Amer. Joum. Sci. A>ts," 1873, p. 407. 



Fig. 37. — Hipposyus. Fig. 38. — Ihjpoliippus. 



