British species of Cyclas and Pisidium . 307 



ginea omnino incrustata : umbones simplices projectura nulla, plerum- 

 que obtusiusculi. 

 Var. I. suboblonga margine dorsali subrecto, minus inaequilateralis, ven- 

 tricosissima, margine basali obtusissimo. 



This species was originally discovered by Professor Henslow, and 

 sent by him many years since to Dr Leach, who gave it the above 

 name. I have since met with it in plenty throughout Cambridgeshire, 

 and likewise in other parts of the county. In fact it is one of the 

 most common species *, inhabiting rivers, ditches, and likewise the smallest 

 streams : it is rather active in its habits, frequently ascending the sides 

 of the vessel in which it is confined, but I never observed it to Avalk 

 along the under side of the surface of the water. The siphonal tube 

 assumes a variety of appearances even in the same individual, and it 

 is very interesting to watch, under a low power of the microscope, the 

 striking and rapid changes of form through which it passes in a short 

 time. It is altogether a variable species, and the shell is of a very 

 different character in different situations ; yet from the circumstance of 

 my possessing many intermediate specimens, I feel confident that the 

 above are only varieties. — Var. c. is from a pond on Bookham-Common 

 in Surrey ; the others are all of frequent occurrence, and are often so 

 much incrusted over with a kind of ferruginous earth as to be entirely 

 concealed, and to present more the appearance of seeds or small lumps 

 of dirt than that of shells. Whether this is the effect of soil and 

 water, as seems to me more likely, or has any thing to do with the 

 habits of the species, is not very obvious. 



• The discovery of this and some other minute species, which though of frequent 

 occurrence remained for a long time unnoticed by Conchologists, may be attributed to the 

 use of a peculiar net invented by Professor Henslow about the year 1815. This instru- 

 ment being constructed of the finest wire gauze enables the collector to strain the water 

 more thoroughly than by any other method previously attempted: and thereby to separate 

 the very smallest shells from the mud in which they are immersed. 



UQ2 



