96 Miscellaneous. 



mon investing integument ; this fifth form may be represented in 



this manner, Khr ^ is P lain > from the foregoing, that at no very 



remote period the two forms which are now bolicved to characterize 

 distinct genera were probably one and the same. The three-toed 

 form {Murcenopsis) is said to be confined to the Southern United 

 States, whilst the two-toed form (Amphiuma) is more widely dis- 

 tributed, extending further north and also embracing the distribu- 

 tion of the former. The digital formula of (1) is Amphiuma in the 

 front pair, and Murcenopsis in the hind pair of limbs ; that of (2) 

 is Amphiuma on one side in the hinder pair, and Murcenopsis on the 

 other. Normal individuals of both genera also occur, as in (3) 

 and (4) ; while (5) represents the beginning of the differentiation 

 of a third generic type, if the number of digits be good and sufficient 

 to characterize genera. Prof. Cope, who has probably handled 

 more specimens of Amphibia than any other American naturalist, 

 informs me that he thinks these variations very uncommon, as he has 

 never in his experience met with any instance in which there was 

 as much variation in the number of digits as exhibited in these 

 Biloxi specimens. They can hardly, however, be regarded as mon- 

 strosities, as the percentage of varying specimens in this series is 

 entirely too high. I am inclined to believe that they are simply 

 instances on the one hand of reversion towards a still older and 

 more unspecialized type, and on the other of a tendency to become 

 specialized or reduced, as in the case where the two digits are 

 covered by a common tegumental investment. If the distribution of 

 species will in any case serve to throw light upon the differentiation 

 of genera, I think that in this instance we may assume, with much 

 show of reason, that the individuals most remote from the centre of 

 maximum development of species and individuals exhibit the greatest 

 tendency towards digital reduction. The most northern form, Am- 

 phiuma, seems to be constantly didactyle, whilst the more southern 

 forms are both di- and tridactyle, which would seem to indicate 

 that the forms most remote from the centre of distribution have been 

 under conditions tending to produce didactylism synchronously with 

 di- and tridactylism at the centre aforementioned. This, however, 

 is only a hypothetical view of the case. 



The admission of Murcenopsis and Amphiuma to generic rank, on 

 account of a difference which is here shown not to be constant, is 

 doubtful. The digits, which, from the fact of their having under- 

 gone reduction, seem to be not so much rudiments as vestiges of 

 former digits, render the legitimacy of the distinction even more 

 open to question ; for I think it cannot be doubted that such a 

 tendency to degenerate, accompanied with a consequent tendency 

 to produce synthetic characters, shows clearly that nature has not 

 yet concluded that they shall be genera, notwithstanding the dicta 

 and definitions of systematists. — Proc. Acad. Nat. Sci. Philad. 

 Jan. 28, 1879. 



