44 JOURNAL OF GONCHOLOGY. 



ON SOME BRITISH VARIETIES OF LIMNJEM. 

 By SYDNEY C. COCKERELL, M.C.S. 

 I have in my cabinet a few interesting forms of Limnsea 

 which it may perhaps be worth while to record. This is, in my 

 opinion, the most engrossing of all British genera on account of 

 the enormous variation in size, thickness, contour, &c., which 

 the commoner species undergo. Scarcely any two examples 

 from different localities are alike ; and the gradations which 

 unite one species to another are so numerous and imperceptible 

 that one is often at a loss to know where to draw the line 

 between closely allied forms. Every conchologist must have 

 met with specimens of Z. auricularia var. acuta on the one 

 hand, and Z. peregra var. labiosa on the other, which he has 

 found it difficult to refer with perfect confidence to either 

 species. Some Z. palustris of the conica type bear a greater 

 resemblance, both as regards habits and appearance, to Z. trun- 

 catula than to the parent stock ; small examples of the var. 

 elongata Moq. can scarcely be distinguished from Z. glabra \ 

 var. corvus is like the stunted Z. stagnalis ; and the vars. tincia 

 and globosa are in extreme cases very similar to aberrant forms 

 of Z. peregra. 



The exact causes of all this variation are at present very 

 much matters of speculation. We know that running water 

 tends to foster a light and slender form of shell, and stagnant 

 water a stronger and more expanded one, but why, for instance, 

 a small pond at Chislehurst should produce among its stunted 

 inhabitants five scalariform Z. stagnalis, or another small pond 

 in the neighbourhood of Tooting a proportion of sinistral Z. 

 peregra, is a very difficult and debatable problem. The same 

 causes act in a similar manner on all the species, and we find, 

 as might be expected, corresponding varieties in most of them. 

 Thus, to omit albinisms and variations in size, the var. reflexa 

 of Z. auricularia, the wzx. labiosa of L. peregra, and the vax. 

 labiata of Z. stagnalis are all parallel. I have moreover a 



.I.e., v., April, 18S6. 



