296 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
VII. Polydactylism in the Equidae. 
A. LITERATURE. 
The anatomy and diseases of the horse have been studied almost as 
thoroughly as those of man, and consequently we find that polydactylism 
in the Equidae has received considerable attention. Aside from the classi- 
cal allusion of Suetonius (’86) to the horse of Julius Caesar “which had 
feet that were almost human, the hoofs being cleft like toes,” the first 
account of polydactylism is that of Winter (1703), who describes two cases. 
Geoffroy St. Hilaire (’32-37) records a foetus which was polydactyle 
in the fore feet, the left foot bearing three nearly equal digits, and the 
right two. Numerous instances have since been noted, the more im- 
portant descriptions being those of Arloing (67), Wood-Mason (71), 
Marsh (’79, ’92), Ercolani (81), Boas (85), Piitz (89), and Ewart (94). 
Blane (93), and Bateson (’94) review the general subject. 
The normal functional digit of the Equidae is m of the typical 
mammalian manus; it consists of a long metacarpal bone and three 
phalanges. The ungual phalanx is completely enclosed in a massive 
hoof. Two splints, representing the metacarpals of digits 1 and Iv, 
articulate at each side of the cannon bone posteriorly and with the 
carpus. The trapezium is a small pea-shaped rudiment lying posterior 
to the trapezoid and often wanting. The os magnum is very large, and 
with it, chiefly, the cannon bone articulates. 
The polydactyle cases cited by various investigators fall into two 
groups, the first of which may be subdivided into three: 
(1) Supernumerary digits representing the development of digital 
! vestiges. 
a. Three metacarpals, the extra digits beng borne on 1 and tv. The 
condition of an extra digit borne on metacarpal 11 may occur on®all four 
feet (Marsh, ’92) or be limited to the manus (Arloing, 67). The extra 
digits are always smaller than 111 and do not function in locomotion ; 
this condition is of quite frequent occurrence. A single case is cited by 
Wood-Mason (’71), in which an extra digit of three phalanges occurs on 
metacarpal 1v ; the radial splint bone (11) was also somewhat better 
developed than in a normal manus. Cases of thiree digits (both 1 
and iv being developed) are cited by Geoffroy St. Hilaire (’32-37) and 
Marsh (’92), but no good anatomical descriptions are given. 
