256 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



('93) figures a single case in which the hallux was developed and 

 duplicated. 



In Loth the cat and dog the normal manus is composed of five 

 digits, but the pollex is much reduced in size. In the pes only four 

 functional digits are present, the liallux being represented by merely a 

 rudiment of metacarpal i. These animals are therefore tetradactyle in 

 the pes, and it is there only that we may look for evidence of reversion, 

 unless we assume the existence of a hexadactyle ancestor. 



Most of the digital abnormalities in Carnivora occur on the radial 

 side of the manus or pes ; digits ii-v remain practically normal in all 

 cases. This is an important fact when the polydactyle conditions in 

 other animals are considered, for it shows that the digits which vary are 

 in most cases those which have been either reduced or modified in the 

 course of phylogenetic development. 



In the pes of the cat the digital abnormalities fall into three classes : 



(1) Five digits, each possessing three phalanges (Fig. B). 



(2) Six digits, five of them possessing three phalanges each, the sixth, 

 which resembles a normal pollex (Fig. CT), exhibiting only two. 



(3) Six digits, each having three phalanges. This is the condition of 

 most frequent occurrence ; the digits in this case are usually so formed 

 that the pes is bilaterally symmetrical. Bateson lays considerable stress 

 upon this symmetrical condition, which is brought about in the following 

 manner. Tlie distal phalanges of the normal extremities are retractile, 

 and are always drawn back to the ulnar side of the second phalanx (that 

 is, in the right extremity to the right, and in the left to the left). For this 

 retraction the second phalanx of each digit is hollowed out on the ulnar 

 side. The supernumerary digits, however, do not conform to this plan, 

 but their ungual phalanges are drawn back to the other (radial) side of 

 the manus or pes ; consequently the second phalanx is hollowed out on 

 the radial side to con-espond. This change in the symmetry of the 

 phalanges may extend also to the second digit (11). 



In the manus of the cat we find the same three types of poly- 

 dactylism and in addition a fourth type, in which there are seven digits 

 present. Digits ii-v are always normal; on the radial side of 11 are 

 three extra digits, the most radial of which is amorphous (Bateson, 

 '94, Fig. 86, p. 319). Torrey (:02) describes a similar case in which 

 seven digits appeared, but the most radial was resorbed soon after birth. 

 In the case described by Howe (:02) three complete extra digits were 

 developed, which he considers similar in structure to digits iii, iv, and v. 

 To this class belong the majority of pijlydactyle cats. When six meta- 



