fl 



l^HE Nautilus. 



Vol.. XVIT. JANUARY, 1904. No. 9. 



GTJNDLACHIA AND ANCYLTJS. 



BY WM. H. DALL. 



During the last ten years I have frequently announced to acquaint- 

 ances and assistants interested in conchology, my belief in certain 

 propositions hitherto unsupported by proof, bearing on the so-called 

 genus Gundlachia, viz : 



1. That Gundlachia is merely an Ancylus which has under favor- 

 able circumstances been able to form a calcareous epiphragm and 

 survive the winter, which ordinarily kills the great mass of individ- 

 uals, and, while retaining the shell of the first year, to secrete an en- 

 larged and somewhat discrepant shell during its second summer. 



2. That not all Ancyli necessarily have the ability to do this, but 

 the practice may have developed in certain small species ; and in 

 tropical regions where the dry season takes the place of winter it is 

 possible that survival may become more or less habitual with some 

 of these s[)ecies, though evidence of this is still needed. 



On no other hypothesis could I account for the fact that single 

 specimens, or a small lot of specimens on a single occasion (after a 

 specially favorable season ?), of Gundlachia have been reported from 

 various parts of the world and described as species, but wiiich no- 

 body has been able to find a second time or in any considerable 

 numbers. Also that the young Gundlachia cannot be distinguished 

 from an Ancylus and usually resembles some common species of 

 Ancylus of the same ponds in which the Gundlachia appears ; wiiile 

 the only species which have been repeatedly collected as Gundlachia 



