0. C. Marsh — Recent Polydactyle Horses. 



347 



(3) The presence of five distinct bones in the second row of 

 the tarsus. One of these is the cuboid fully developed ; next, 

 the external cuneiform bone supporting the great metatarsal ; 

 then three smaller bones on the tibial side. These five tarsals 

 correspond either to those in the reptilian foot, or the first 

 may be regarded as a sesamoid, and .the cuboid, as double 

 (including the fourth and fifth), as it is now usually con- 

 sidered by anatomists. Another explanation may be suggested ; 

 namely, that the inner, pendent bone is a remnant of the first 

 metatarsal. Such a rudiment apparently exists in some fos- 

 sil horses, and its appearance in the hind foot of a recent 

 animal which had the first digit of the fore foot so well repre- 

 sented would not be strange. The bone usually regarded as 

 the inner cuneiform in the existing tapir and rhinoceros may, 

 perhaps, include the same remnant. In the modern horse, this 

 bone is sometimes double, but the specimen represented in 

 figure 9 shows that it may be composed of three elements. 



12 



Figure 12. — Left fore foot of Rhinoceros bicornis, Linnjeus. 



Figure 13. — Left hind foot of same animal. One eighth natural size. 



The frequent reappearance of the second digit as an extra 

 toe in the modern horse would seem to indicate that this 

 feature was functional in a late ancestor, but no fossil equine 

 with two toes has yet been found.* 



The presence of four toes in the fore feet, each supported 

 by its true carpal bone, is a fact of much importance, and is 

 clearly a case of reversion. The four distinct cuneiform bones 



* A small species of Pliohippus from the Pliocene of Oregon may be an excep- 

 tion. An incomplete hind foot in the Yale Museum shows the second metatarsal 

 as a splint bone, the third very long and slender, and the fourth so well developed 

 that it probably supported phalanges. This may also be a case of reversion. 

 The species is new, and may be called Pliohippus gracilis. 



