XV. 



ON THE ORIGm OF THE FOOT STRUCTURES OF 



THE UNGULATES. 



The following considerations have been suggested by a study 

 of the primitive types of the odd- and even-toed ungulates. I 

 first, in 1874, recorded the oi^inion that the Mammalia with a 

 reduced number of digits were derived from pentadactyle planti- 

 grade types.* The ungulate order which fulfills this requirement 

 is the Amhlypoda, and from them, I doubt not, both the Perisso- 

 dactyla and Ai'tiodactyla have arisen, although not from any of 

 the genera now known. Both of these great orders display a 

 regular diminution in the number of the digits ; in the former, 

 by reduction and extinction on both sides of the third digit ; in 

 the latter, by reduction and extinction on each side of the third 

 and fourth digits. Mr. John A. Ryder f has pointed out that 

 reduction in digits is probably directly related to strains and im- 

 pacts. He reminds us that the anterior digits are reduced \\\ 

 Mammalia of unusual scansorial or fossorial powers ; while in 

 forms which display powers of running, the reduction is seen first 

 in the posterior feet, which propel the body much more than the 

 fore feet. This view is well illustrated in the Perissodactyle 

 families, the majority of which have the digital formula 4 — 3. 



No reason has ever been suggested, so far as I am aware, in 

 explanation of the fact that one series of ungulates has retained 

 two digits, and the other only one ; that is, why there should 

 have been two kinds of digital reduction instead of one kind. In 

 seeking for an explanation, we will remember tliat the tarsus in 

 the odd or single-toed line is bound together by fixed articula- 

 tions, while in the cloven-footed line it is interrupted by the hinge 

 between the first (astragalus) and second rows of bones. The 

 hinge-joint, being more liable to luxation than the fixed articula- 



* "Journal Academy Philadelphia," March, 1874. 

 \ "American Naturalist," October, 1877. 



