No. 531] GUNDLACHIA AND ANCYLUS 



181 



formed shell is yellowish translucent, while the older 

 part, especially when the pond or aquarium has a muddy 

 bottom, often becomes darkened or even blackish, and 

 more or less covered by a growth of conferva. Mr. 

 Allen calls attention to the fact that the sharp line of 

 demarkation which separates the dark encrusted shell 

 of the septate from its translucent Gundlachia extension 

 in the final stage, is evidence that the growth is not con- 

 tinuous, but that a resting period of some duration sepa- 

 rates the two stages. 



I have preferred for the most part to refrain from 

 theorizing on the inferences to be drawn from the data, 

 letting them speak for themselves. To me, however, the 

 facts tend strongly to confirm the hypothesis suggested 

 in the opening paragraphs of this paper. 



General Notes 



The following notes are partly summarized from a 

 rather voluminous correspondence with Mr. Allen, ex- 

 tending over more than four years. 



The Thornburg lagoon is an abandoned channel of 

 the Cuyahoga Eiver. In 1903 the river was fairly well 

 stocked with Unionidae, but soon after that date the con- 

 tamination of the river by drainage and sewage killed off 

 the naiad population. This contamination is not believed 

 by Mr. Allen to have seriously affected the water of the 

 lagoon, though for some reason it does not seem to be a 

 place favorable to vigorous growth of mollusks. It pro- 

 duces a dwarf Piano rbis parvus, a poorly developed 

 Physa, a small form of Lifmncea hu mil is modicella and a 

 scanty supply of Ammcola. It is nearly filled with 

 Nuphar on the leaves of which Ancylus is found; also 

 Ceratophyllum, Potamogeton, etc., occur, especially 

 where the water is shallow. 



At one place the bank bordering on the lagoon is steep 

 and the water near it deep, so here even at low water 

 mollusks would never be left dry. There is another por- 

 tion of the lagoon where a wide zone, producing vegeta- 



