ORIGIN AND DEVELOPMENT OF THE EXISTING HORSES. 



73 



is greatly enlarged and somewhat flattened while in Hippidium they are almost 



equal. 



It should be here remarked that the genus 

 which holds direct antecedent relation to Hippoiher- 

 ium is Paloplotherium, Owen. Here the anterior 

 internal lobes of the superior molars are distinct and 

 there is a slight deposit of cementum in the valleys. 

 The additional lobe represented by / in Fig. 12, 

 however, is very rudimentary if not in some instances 

 entirely wanting. The ancestral hne of Protohippus 

 on the other hand is indicated by Mesohippus and 

 Anchithcrium, in which the inner lobes of the superior 

 molar are connected with the anterior and posterior 

 cross-crests. 



Some species of Hippotherium exhibit an enlarge- 

 ment of the antero-internal lobes of the upper molars 

 almost equal to that oi Equus with a marked tend- 

 ency to confluence. A new species from the Loup 

 Fork beds of Oregon which may be called Hippo- 

 therium sinclairi, exhibits these characters of the 

 superior molars in common with H. occidentale. It 

 may be distinguished from the species, however, by 

 its small size and the less marked concavity of the 

 inner contour of the antero internal lobe. This 

 c haracter reaches its maximum development in Equus 

 (Fig. 23 ai). An arrangement of some of the 

 species of Hippotherium to show the extremes with 

 reference to their approach or departure from the 

 typical Equus caballus in their characters would be 

 as follows : 



H. Paniense. — Anterior internal lobe cylindric 



and equal in size to posterior internal lobe. Plications of the enamel borders 



of the valleys few. 



H, seversum. — Anterior internal lobe pear-shaped with anterior angle bent 



inward to meet corresponding fold on cross-crest. Antero and postero-internal 



lobes sub-equal. 



H. sinclairi. — Antero-internal lobe slightly concave on its inner border, and 



elongated fore and aft; much larger than posterior internal lobe. 



H. occidentale. — Species large. Enamel plications many. Anterior internal 



lobe greatly elongated fore and aft, crescentoid in section with concavity directed 



inward. Anterior cross-crest throws out one and sometimes two folds toward 



this lobe which almost connect it with cross crest. 



Fig. 21. 



Right posterior foot of Protohippus 

 sejunctus, Cope ; one-half nat, size. 



