1879.] NATURAL SCIENCES OF PHILADELPHIA. 15 



tions 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 monstrosities, as the percentage of varying 

 specimens in this series is entirely too high. I am inclined to be- 

 lieve that they are simply instances on the one hand of reversion 

 toward 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 ease 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, Amjihiuma, 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 con- 

 ditions tending to produce didaetylism synchronously with di- and 

 tridactylism at the centre aforementioned. This, however, is only 

 a hypothetical view of the case. 



The admission of Mursenopsis 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 ten 

 dency to produce synthetic characters, shows clearly that nature 

 has not yet concluded that they shall be genera, notwithstanding 

 the dicta and definitions of systernatists. 



