MOLLUSKS. 319 



of mollusca were in company with the above limneus ,- 

 but this species was the most abundant, and, from 

 the circumstance of its being usually accounted rare, 

 the most interesting of all. Many of the specimens 

 were very large, much exceeding in size any I have 

 seen in collections. These shells, however, did not 

 prevail in any great numbers after that year. A few 

 continued to shew themselves for three or four sea- 

 sons, but they gradually disappeared ; and now many 

 years have elapsed since I noticed even a single indi- 

 vidual.* 



A somewhat similar phenomenon, in the case of 

 another species, occurred in February 1825. The 

 early part of that month had been very wet, causing 

 the water to stagnate in large puddles in several parts 

 of the park at Bottisham Hall, but which parts are 

 not usually flooded, though sometimes a little swampy. 

 Happening shortly afterwards to cross the park with 

 a shell-net in hand, I immersed it into one of these 

 puddles casually as I passed, when, to my surprise, 

 I drew it out full of the Aplexus hypnorum^ a species 

 which I had not at that time taken before in Cam- 

 bridgeshire, though I have since met with it in one 

 or two places. In this puddle the shells were col- 

 lected in immense quantities, whilst none of the 

 other puddles contained one. The shells were of 

 various sizes, though none were full-grown. It were 

 almost vain to speculate as to how they came there. 



* I observe that Gray, in his Manual, speaks of this species as 

 periodically abundant in certain places, which seems 10 be con- 

 firmed by what is above stated. 



f Gray's Manual, p. 255, torn. ix. f. 113. 



