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THE AMERICAN NATURALIST. [Vol. XXXVII. 



in Chimarrogale, or in addition to the fringes the foot may be 

 widened by disc-like pads and at the same time be webbed as in 

 Nectogale and Myogale. 



Another transformation due to life in the water is found in 

 the shortening of the arm and forearm and lengthening of 

 the digits. This begins in the Pinnipedia where the arm is con- 

 siderably reduced in length though it is still serviceable to some 

 extent in locomotion and in most cases capable of supporting 

 the weight of the body. In the Sirenia and Cetacea, where 

 progress upon the land has been entirely given up, the arm is 

 still more reduced and in the latter group may be entirely with- 

 drawn into the body wall, only the hand remaining outside to 

 form the fin. In both these groups, but in the latter especially, 

 the arm no longer functions as a propelling organ but serves, as 

 in most fishes, merely as a balancing organ, the greatly devel- 

 oped tail furnishing the motive power. The extreme of adapta- 

 tion in the hand is reac hed in the addition of extra phalanges in 

 the digits, hyperphalangy, and the addition of an extra digit, 

 hyperdactyly, thus increasing the extent of the hand. Hyper- 

 phalangy is common among the Cetacea, where as many as 

 twelve phalanges may occur in a single digit, and even a greater 

 number than this in one species, Globiocephalus melas. It 

 apparently does not occur in other aquatic mammals except 

 occasionally an extra phalanx may be found in Sirenia, but a 

 close parallelism is seen in the ichthyosaurs and plesiosaurs. 

 Kukenthal accounts for hyperphalangy as a result of retarded 

 ossification and the formation of double epiphyses. These 

 epiphyses tend to ossify at later and later periods and finally 

 to become entirely separate bones forming the extra phalanges. 

 As compared with other theories that have been advanced to 

 account for hyperphalangy, this theory of Kukenthal, strength- 

 ened by the array of facts which he brings to its support, 

 seems most reasonable and sufficient to account for the condi- 

 tions in the Mammalia at least. Hyperdactyly is not common 



Cetacea, e. g. Delphinaptcrus leucas, the white whate^ by a 

 splitting of the fifth digit, as shown by Kukenthal and Leboucq. 

 In the ichthyosaurs the process went much farther, several sec- 

 ondary digits being formed. 



