256 THE HORSE AND ITS RELATIVES 



like those of Protohippus, are functionally one-toed. 

 The genus, which ranges in space from Texas to 

 Oregon and Montana, is typically represented by 

 P. insignis. 



In this place it will be convenient to notice 

 certain horse-like animals which, although attaining 

 very considerable specialisation, evidently form a 

 side-branch, and are off the ancestral line of the 

 modern representatives of the horse family. The 

 group is typified by Hipp avion gracilis of the Lower 

 Pliocene strata of Germany, Greece, Spain, and 

 other parts of Europe ; but is represented in the 

 corresponding formation of India by H. theobaldi^ 

 and by other species in Persia and China ; all of 

 these being animals of the approximate size of a 

 Galloway, but with a shorter head and a deep 

 depression in the skull in front of the socket of the 

 eye, probably for the reception of a lachrymal 

 gland. The lateral toes are rather larger than in 

 Protohippus^ but as they scarcely reach below the 

 lower end of the first phalangeal of the main digit, 

 they could have been of little or no functional 

 importance. The most characteristic feature of 

 Hipparion is, however, as shown in B of the illus- 

 tration on page -^y-}^, that the anterior inner pillar 

 (or protocone, as it is often called) is completely 

 surrounded by a ring of enamel, and is thus entirely 

 cut off from the rest of the crown. In some 

 specimens there are, however, little projections 



