No. 531] GUNDLACHIA AND ANCTLUS 



187 



small proportion of the individuals advance beyond the 

 septate stage; and also that, of the ancyloids, only part 

 reach that stage. It is also probable, from Mr. Allen's 

 observations, that anything which tends to retard de- 

 velopment may coincidently increase the tendency to 

 form a septum. 



Since there is a period of least activity in May, a nat- 

 ural observation year will be from one May to another. 

 Mr. Allen summarizes the results obtained during the 

 period, May, 1908, to May, 1909, as follows : 



None being taken before July nor after the following 

 March, there were secured between July, 1908, and 

 March, 1909, inclusive: 



From August 19, 1909, to December 13, 1909 : 



Further correspondence, during February, 1910, af- 

 fords additional notes. 



A lot of the wild Thornburg Ancylus in alcohol was 

 sent by Mr. Allen and, contrary to his expectation, on 

 careful comparison with his series of ancyloids from his 

 aquaria, no difference, beyond slight individual varia- 

 tions, could be observed in the shells of the two series, 

 while the radula and the soft parts, after repeated com- 

 parisons, seemed to be identical in both. 



Mr. Allen especially notes that in the winter, 1909-10, 

 the septates were the prevailing form in his aquaria, 

 exactly the reverse of the case when the aquaria were 

 freshly established. The generation, which appeared in 

 May and June, 1909, in the " Planorbis-free' ' jar, was 

 dwarfed was not in sight during the latter part of the 

 winter, 1909-10, and may possibly have all died. Mr. 

 Allen attributes the poor success of this jar to the use of 



