82 



ON THE HOMOLOGIES AND ORIGIN OF THE 



5. PalcEOtheriodont type. — Immediately following the form of the Palceos^ops 

 molar we have that of Hipposyus* where intermediate tubercles stand between 

 ^. ,, the inner subconic and the exterior longitudinal ores- 



centoid tubercles. They are compressed so as to be 

 ^« transverse, and only need more complete connection 

 with the adjacent tubercles to give the oblique trans- 

 verse ridges of Anchitherium, PalcBotherium, and Hyra- 

 Hi-pposyus. codon, Rhinocerus, etc. Hipposyus was originally com- 



pared with Anchitherium by Dr. Leidy, and the writer in ignorance of his language 

 remarked:-}- "An interesting annectant form is seen in Orohippus procyoninus, 

 where the two intermediate tubercles which separate the inner cones from the 

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

 pair of transverse ridges which disappear in front of the bases of the outer Vs. 

 They represent the oblique crests, of Palcsotherium and Anchitherium, and thus the 

 genus Orohippus furnishes a station on the line from Palcsosyops to the horses." 

 Contemporaneously and quite independently Prof Marsh expressed similar viewsj 

 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. 25). The elevation of the tubercles and deepening 

 of the valleys gives us the Selenodont type of superior molars again in this genus ; 

 while the lower molars only differ from that type in having the crescents alternate 

 instead of opposite, forming the Hippodont pattern (Fig. 26). There can be little 



Fis. 23. 



Fig. 24. 



Fig. 25. 



Fig. 26. 



Hipposyus. 



Hypohippus. 



Equus. 



doubt that the line of the horses comes through Hipposyus from the Bunodonts, 

 rather than through Palaotherium, as has been suggested by some writers. 



* This genus was named by Leidy June, 1872 (Philada. Acad., p. 37); August 3d, 1872, the writer called it 

 Helofherium; and Aug. 7th, 1872, it was named Orohippus by Marsh, etymology from 'irfrtoj and Svj, hence 

 distinct from Htppohi/us G. and F. 



t Hayden's Geol. Surv. Montana, etc., 1873, p. 647. 



X Amer. Journ. Sci. Arts, 1873, p. 407. 



