Rev. L. Jenyns on the smaller British species of Pisidium. 105 



ceived that one of the smaller specimens from Ambleside did 

 possess the appendage in question, though not quite so much 

 developed as in the Cambridgeshire ones in my collection. This 

 satisfies me that the appendage is not an essential character; 

 and there being scarcely any other character, except greater size, 

 by which the P. Henslowianum can be distinguished from the 

 P. pulchellum, I am inclined to the opinion that these two con- 

 stitute but one species, the former being only one of the nume- 

 rous forms which this variable shell is liable to assume. There 

 is no difference in the animal, nor in the habits of the two kinds 

 respectively. If the P. Henslowianum, from any other yet to be 

 discovered characters, be really distinct, I conceive that the 

 normal form is rather the variety without appendages than the 

 one with them, as the former seems the more plentiful of the 

 two, and includes the largest specimens I have seen. Possibly 

 the appendages are liable to wear off with age, especially in 

 running water, where they are necessarily exposed to more 

 friction. 



Pisidium pulchellum. — There are three principal varieties of 

 this species described by me in my monograph. For the first of 

 these, — under an idea expressed formerly by myself, that it 

 might constitute a distinct species, — Dr. Gray, in his edition of 

 Turton's ' Manual of the Land and Freshwater Shells of the 

 British Islands/ did me the honour to propose the name of 

 Jenynsii. But I do not believe now that this variety is any 

 more distinct from the var. /3 of my monograph, than both these 

 varieties, as well as others not calling for any particular notice, 

 are distinct from the P. Henslowianum. And as the latter name 

 is of prior date to pulchellum } in respect of publication, I should 

 propose that it be the one in future adopted as the general name 

 for this species with its many varieties, more especially as it is 

 to Professor Henslow that we owe the original discovery of both 

 the species now brought together. 



The variety which Dr. Gray has termed Jenynsii differs from 

 the more ordinary form of pulchellum by the striae being more 

 deeply cut, and the shell being broader in proportion to its 

 length. But, as regards the variation of the strise, we have a 

 somewhat parallel case in the two varieties of P. amnicum noticed 

 in my monograph, and formerly considered by Dr. Leach as two 

 distinct species, under the respective names of Per a fluviatilis 

 and Pera Henslowiana. There can be no doubt that these last, 

 of each of which I have specimens from the identical streams 

 from which Dr. Leach received his, are mere varieties ; nor is 

 there any more doubt, in my opinion, that Jenynsii and pulcheU 

 lum are likewise mere varieties of one species, dependent upon 

 water and other local circumstances. 



