292 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



7. Tlie extra digits articulate witli the trapezium in nearly every case ; 

 they therefore represent the development of a vestigial pollex, hut may 

 vary extremely from the normal pollex structure. 



8. There may be cases where the extra digit is formed by the dupli- 

 cation of digit II, but there is strong evidence against this being the 

 general rule. 



9. Two supernumerary digits may be formed by the duplication of 

 the vestigial pollex ; there are no grounds for considering one of them a 

 *'prae-pollex." 



VI. Polydactylism in Ruminants. 

 A. Literature. 



Observations have been made on polydactylism in ruminants and 

 descriptions given by Geoffrey St. Ililaire ('32-37), Goodman ('68), 

 Cliauveau et Arloing ('79), Boas ('90), Baumuller ('92), Blanc ('93), 

 and Bateson ('94). In the normal manus of ruminants, in and iv are 

 the functional digits, and in all forms save the water chevrotain their 

 metacarpals are fused to form a single " cannon bone." The pollex is 

 always wanting; digits il and v are reduced in varying degrees in the 

 different groups of ruminants. In the camel they are wanting ; in the 

 ox metacarpal v remains as a proximal rudiment ; the phalanges are 

 completely gone, but a " dew-claw " repi'esents each hoof. The sheep 

 has the two distal phalanges and hoofs of ii and v persistent, while in 

 the Cervidae these digits are represented by three well-developed pha- 

 langes and the distal ends of the metacarpal bones; the hoofs of digits 

 II and V are functional when the deer is running or travelling over soft 

 ground. In the water chevrotain there are four complete digits, each 

 formed of a distinct metacarpal and three phalanges. 



I know of no instance of polydactylism in the camel, and there are 

 few descriptions of such abnormalities in sheep. Geoffroy St. Ililairo 

 ('32-37) describes the manus of a lamb in which digits i, ii, and v were 

 developed ; digits i and ii were borne on the same metacarpal and 

 probably represent a duplicated condition of digit ii. The best de- 

 scription of polydactylism in the sheep is that of Chauveau et Arloing 

 ('79). The manus of a lamb is figured, in which both the second and 

 fifth digits are developed, each being composed of a distinct metacarpal 

 element and three phalanges nearly as large as those of the functional 

 digits. This condition is certainly due to the development of vestiges, 

 and has been attributed to reversion. 



