O. C. Marsh — Recent Polydactyle Horses. 



549 



In the horse and its allies, recent and extinct, the axis of 

 each foot passes through the middle of the main, or third, digit, 

 while in all the so-called even-toed mammals, the axis is always 

 along the side of this digit. This is the essential difference 

 between the feet of the two groups. 



The names Mesaxonia (middle axis) and Paraxonia (side 

 axis) proposed by the writer in 1879,* as substitutes for Owen's 

 terms, are more exact definitions, but thus far have not met 

 with general favor, for it seems almost impossible to dislodge 

 an established error in science. The term " coral insect," for 

 example, of the older writers, still maintains its place in 

 popular science, and even in some modern text books. 



16 



14 



/v 



Figure 1 4. — Fore foot of oldest fossil horse, Eohippus pernix, Marsh. 



Figure 15. — Hind foot of same. Lower Eocene. 



Figure 16. — Fore foot of Orohippus agilis, Marsh (type). 



Figure 17. — Hind foot of same. Middle Eocene. All left feet, one half natural size. 



In explanation of the extra digits of the recent horse, two 

 views have been advanced : (1) that they are reduplications 

 of the main digit, like the occasional sixth finger of the 

 human hand, or result simply from a division of this toe ; (2) 

 that they are true cases of reversion to a polydactyle ancestor. 

 The former view was long held by writers on the subject, and 



* This Journal, vol. xvii, p. 501, June, 1879. 



