44 



Valvata piscinalis (The Stream Valve Shell). Muller. 

 Figures 20 and 21. 



This abundant fresh-water shell assumes two very 

 distinct forms, so different in appearance from each 

 other that formerly they used to be recorded under two 

 different names. The more stunted form of which was 

 the Valvata depressa of Pfeiffer. From an examina- 

 tion of many specimens, Nilson Grey, and Forbes have 

 each declared the V. depressa to be nothing more than 

 a variation in the shape of Valvata piscinalis. In the 

 one form the convolutions are produced and the um- 

 bilicus comparatively small, while in the other the con- 

 volutions are depressed, and having a large umbilicus. 



There are five convolutions, which are somewhat 

 polished, and rather pellucid. The operculum is deep, 

 and tolerably large, and the convolutions are very 

 rounded, the apex being small, but very blunt. The 

 aperture is round and large. It is pale horn-coloured. 

 The size is rather less than a quarter of an inch in 

 diameter, and about a quarter of an inch in height. 



In one locality in this neighbourhood (the Musco-sic 

 dyke) the shells are mostly covered over with patches 

 of a calcareous deposit, which gives them a mottled 

 appearance. I have not found this deposit on speci- 

 mens taken from any other water besides this brook, 

 and even here only over a space of fifty yards. 



Valvata piscinalis was first described by Petiver. in 

 his'* Gazophylacium," under the name of Valvata ohtusa. 



M. Bouchard ' Chautereaux describes this shell as 

 laying, between the months of May and August, from 



