No. 531] 



GUNDLACHIA AND ANCYLUS 



I 39 



My last communication from Mr. Allen, dated De- 

 cember 11, 1910, contains the following- additional notes: 



As I have already written there was plenty of A-forni (ancyloids) 

 and no G-fonn (septates) visible in my original lar-e jar last summer. 



stages, youii"' septates have been visible in fair al)iindanee. I counted 

 about a dozen in siirht at one lime. 



He concludes that ancyloids are present most of the 

 year, but only young ones in May and mostly also in June. 

 But septates appear to be a strictly winter form, that is, 

 the immature septate stage appears in August or later, 

 reaches maturity (Gundlachia) in February or March, 

 and disappears about the end of April, after which and 

 a shorter or longer interval the young ancyloids of the 

 season begin to appear in the jars. 



If the hypothesis stated at the beginning of this paper 

 be well founded, it would explain why mature Gund- 

 lachias appear, if at all, usually as a few individuals in 

 any given locality, and their presence can not he counted 

 on, as in the usual case of fresh-water mollusks, and is 

 distinctly a rarity in the temperate regions of the conti- 

 nent, where there are no well defined wet and dry seasons. 



