188 THE AMERICAN NATURALIST [Vol. XLV 



swamp soil from a different place from that previously- 

 used. 



February 15, 1910, being a dark day and therefore 

 favorable for the septates to be out of sight, Mr. Allen 

 counted those visible in the large aquarium. Six sep- 

 tates and one ancyloid were noted. This illustrates the 

 observation that (excepting the " Piano rbis-free" jar) 

 the septate is the prevailing form this season, and is 

 promoted by causes which dwarf or retard growth. 



After noting the inexplicable way in which fresh- water 

 mollusks sometimes appear and disappear from pools 

 where they occur, Mr. Allen further suggests that the 

 septate form may be a prelude to total disappearance of 

 the species from a given place. 



Another count on February 17, 1910, gave three ancy- 

 loids and three septates in sight, which Mr. Allen re- 

 marks is the first time for a considerable period that the 

 two forms have appeared in equal numbers. In the large 

 jar every mature specimen seen this season has been 

 conspicuously bicolored, the ancyloid or septate part 

 being stained deep black, while the flaring extension is 

 translucent and colorless, indicating that a resting period 

 intervened between the completion of the septum and the 

 formation of the mature shell. 



Three ancyloids seen February 17 were all translu- 

 cent and about the same size. There can be little doubt 

 that they date from the summer of 1909. Hence, Mr. 

 Allen infers that the blackened original shells of the ma- 

 ture Gundlachia date from the season previous. 



