350 



0. C. Marsh — Recent Polydactyle Horses. 



is still maintained essentially by some eminent anatomists.* 

 The large number of cases, however, now investigated by 

 the writer make it probable that, for most instances, at least, 

 the second explanation is the true one. 



In the case of horses, recent and extinct, the question of 

 atavism is much simplified, as they start with a pentadactyle 

 form, and each successive change in the modifications of the 

 limbs, and of the feet, and likewise in the dentition, can be 

 traced through a regular series from the early Tertiary down 

 to the present time. Nearly every instance of polydactylism 



Figure 1 8. — Left fore foot of Mesohippus celer, Marsh. 



Figure 19. — Left hind foot of same. One half natural size. Lower Miocene. 



Figure 20. — Left fore foot of Miohippus anceps, Marsh. 



Figure 21. — Left hind foot of same. One third natural size. Upper Miocene. 



* Flower, Osteology of the Mammalia, p. 296, 1885 ; and G-egenbaur, Morpho- 

 logisches Jahrbuch, p. 584, 1880. 



