10 FRESinVATER MOLLUSKS. 



met with thein in this canal, why we may not expect to find *L. auri- 

 cularia, a pretty shoU in the shape of an ear, L. stagnalis, the largest 

 British species, and L. tnincatula, making, in all, five ont of the eight. 

 If not in this canal, we must not, I think, at any rate rest satisfied 

 until we have found them somewhere in the neighbourhood. 



4th. Ancijlus, or freshwater Limpet, of which there are two species. 

 Th^y are usually found adhering to stones or water weeds, more com- 

 monly the former, unless found in situations with a very muddy bottom, 

 Oi" the two, they rather prefer clear to stagnant water, although they 

 will live for years in a healthy aquarium. They are both small ; the 

 larger, Ancylus fluviatili*, never exceeding half an inch in diameter. 



This completes my list of those that respire both air and water, and 

 they are all, like the common snail, without an operculum or trap- 

 door with which some moUusks shut themselves in when they retire 

 within their shell. 



We now come to those that are provided with an operculum, such 

 as you must have noticed in the common periwinkle. Of these wo 

 have in England two genera, which, like the common snail, breathe air 

 only ; but these I pass over as foreign to our purpose to-day. The re- 

 mainder (there are but five families) respire water only. 



1st. Of these I pass over one, Dr. Gray's Assiminea, as it has been 

 only as yet found in the River Thames. 



2nd. Bythinia, a genus of pretty mollusks of which there are three 

 species, and of which we must make up our minds to find one at least. 

 The animal when crawling puts forth two white, elegantly-curved ten- 

 tacles, and is, altogether, a genteel looking creature and exceedingly 

 dainty in an aquarium, choosing out for itself all the youngest and 

 most tender bits of water weed. 



3rd. Fiiludtiui : Of this there are two species, not particularly easy 

 to distinguish. 



These are certainly by far the handsomest of our fresh-water sheila. 

 The colour of the shell is a dark olive green with three purple bands 

 running round it ; the animal which is shy and unlike the peacock, not 

 fond of exhibiting itself, is of a rich dark umber tint, covered with 

 minute yellow dots, suggesting the idea of gold dust. 



It has the peculiar characteristic of hatching its young in the ovary, 

 and ejecting them when alive three or four at a time when they are 

 about two months old. Whenever I have found in the breeding season 

 shells of these creatures icith the operculum perfect, in ichich the aninial 

 had 2)erishcd, I have invariably found two or three of the young fry 

 also, about | of an inch in diameter. 



4th. Valvata, of which there are two species. 



This is a small animal generally found adhering to stones and sticks 

 in still and gently running waters. The animal is pretty, being almost 



*This shell was found on the day in which this paper was read. 



