chap, xiil] DIGITS OF HORSE : SPECIAL CASES. 373 



truth of certain views of the phylogeny of the Horse and employ a system of nomen- 

 clature based on these views. This is not retained in the abstract here given.] 



ARTIODACTYLA. 



In the domesticated animals of this order digital Variation is 

 not rare, being in the case of the Pig especially common. Such 

 variation has been seen in the Roebuck and Fallow Deer, but not 

 in any more truly wild form so far as I am aware. These varia- 

 tions may take the form either of polydactylism or of syndactylism. 

 Of the former a few cases are known in Ox, Sheep, Roebuck 

 Fallow Deer, and many cases in the Pig ; syndactylism has been 

 seen only in the Ox and in the Pig. The absence of cases of 

 syndactylism in the Sheep is a curious instance of the caprice 

 with which Variation occurs. 



The phenomena of polydactylism in Pecora may conveniently 

 be taken separately from the similar phenomena in Pigs. 



Polydactylism in Pecora. 



At the outset one negative feature in the evidence calls for 

 notice. It is known that in the embryo Sheep rudiments of meta- 

 carpals II and V exist 1 which afterwards unite with III and IV. 

 In view of this fact it might be expected by some that there would 

 be found cases of Sheep and perhaps Oxen polydactyle by develop- 

 ment of the digits II or V. In the Sheep only one case (No. 

 555) is known that can be possibly so interpreted; and in the Ox 

 there is no such case unless Nos. 557, 558, and 559 should be held 

 by any to be examples of the development of II, a view attended 

 by many difficulties. 



The two following examples are the only ones known to me in 

 which there can be any question of reappearance of a lost digit, 

 but in neither is the evidence at all clear. 

 555. Sheep. Some specimens of a small Chilian breed had an 

 extra digit on the hind foot. It was not present in all individuals 

 and was not seen to be inherited ; but normal parents were 

 observed to have offspring thus varying. [From the description 

 given I cannot tell whether the extra digit was internal or 

 external. Also, though said to have been on the hind foot, in 

 describing the bones the cannon-bone is twice called metacarpus ; 

 probably this is a slip for metatarsus.] The digit was only 

 attached by skin. It contained a bent bone, of which the upper 

 segment was 20 mm. long, the lower 13 mm. Proximally the 



1 Rosenberg, Z. f. w. Z., 1873, xxm. pp. 126—132, figs. 14, &c. Sometimes 

 these rudiments remain fairly distinct at the proximal end of the cannon-bone, 

 especially of the fore foot. See Nathusius, Die Schafzucht, 1880, pp. 137 and 142, 



figs. 



