364 



MERISTIC VARIATION. 



[part I. 



bones" were separate 1 , as shewn in Fig. 107 C. The fore feet of the 

 same animal were in the condition described in (1) b. [See No. 537.] 

 Marsh, O. C, Am. Jour. ScL, xliii. 1892, pp. 340 and 345. 



A B 



as 



Fig. 107. Limb bones of a polydactyle horse. 



A. Left fore foot. No. 537. 



B. Left hind foot. No. 535. 



C. Tarsus of right hind foot from the inside. No. 535. 

 n, navicular, cb, cuboid, 4, ecto-cuneiform. 1, 2, 3, three bones placed 



cuneiforms, td, trapezoid, tm, trapezium, u, unciform, m, magnum. 



I, II, III, IV, numerals affixed to the metacarpals on the hypothesis that these 

 are their homologies. Cp. Fig. 108, which is lettered on a different hypothesis. 



(After Marsh.) 



b. Four metacarpals. 



This condition is a higher manifestation of the variation seen 

 in the cases just given. In No. 533 the digit II was developed and 

 in addition the trapezium had appeared ; in the cases now to be 



1 Marsh introduces this case in support of a contention that these variations are 

 of the nature of Keversion. Upon the same page appears the statement that "in 

 every specimen examined, where the carpal or tarsal series of bones were preserved 

 and open to inspection, the extra digits were supported in the usual manner," I. c, 

 p. 345 : this assertion is hardly in agreement with the previously stated fact that 

 the metatarsal II is supported by two cuneiform bones. On p. 349 Marsh comments 

 on the presence of five bones in the distal row of the tarsus, and from the expres- 

 sions used it is implied that five such bones had been met with in other polydactyle 

 hind feet. A number of alternative explanations are proposed ; (1) that the five 

 tarsals correspond "to those of the reptilian foot"; (2) that the first may be a 

 " sesamoid " ; (3) that the first may be a remnant of the first metatarsal, for such 

 a rudiment " apparently exists in some fossil horses." With conjectures of this class 

 morphologists are familiar. Into their several merits it is impossible to inquire, but 

 it may be mentioned that the real difficulty is not the presence of the cuneiform 

 marked 1, but the fact tbat the tarsal element of the digit II seems to have been 

 double, and that the digits in reality are not supported in the usual manner. 



