112 MOLLUSCA FROM THE CRAG. 
4. Pistptum pusitituM, TZurton. 
TELLINA PusILLA. Turt. Conch. Dict., p. 167, 1819. 
Cycxias pusitta. Turt. Brit. Biv., p. 251, t. 11, figs. 16, 17, 1822. 
_ — Id. Land and F.-Water Shells, pl. 1, fig. 7. 
— appa. Alder. Trans. Nat. Hist. Soc., Newcastle, vol. i, p. 41. 
— Fontinatts. Drap. Moll. Terr. et Fluy. France, p. 130, pl. 10, fig. 8, 1805. 
Evevesta Henstowrana. Leach, MSS., fide Jenyns. 
PIsIDIUM PUSILLUM. Jenyns. Trans. Camb. Phil. Soe., vol. iv, p. 302, t. 20, figs. 4—6, 
1831. 
— —_ Forb. and Hanl. Hist. Brit. Moll., vol. ii, p. 123, t. 37, fig. 10, and 
pl. O, fig. 9, 1849. 
Spec. Char. Testd pusilla, orbiculato-ovatd, compressiusculd, subinequilaterd, subti- 
hssimé striatd ; umbonibus prominulis. 
Shell small and slender, roundedly ovate, somewhat compressed, slightly inequi- 
lateral, very finely striated ; umbones but little projecting. 
Length, 3th of an inch. 
Locality. Clacton, Copford (J. Brown), Grays (Pickering). Recent, Britain, France. 
This species is by no means abundant in my Cabinet from the former locality, 
while Mr. Brown has found it in large numbers, in what is, perhaps, a more Modern 
deposit at Copford. The principal distinction of this shell appears to be its generally 
greater gibbosity ; it is less inequilateral than any of the other species. 
We have thus, it seems, four well determined species in this genus from the 
purely Fresh-water Deposits of this Kingdom, while the living British Pisidia have 
been separated by British Conchologists into not less than eight or nine. I confess, 
not to be very well acquainted with the recent forms, but judging from a general 
knowledge of the variability amongst the fresh-water shells in particular, I think too 
much dependence has been placed upon differences, arising from locality and other 
external causes, and that slight variations resulting therefrom have been considered of 
sufficient importance for the establishment of distinct species; I am inclined to 
belieye, all the forms existing in England might be included in four or at most five 
species. 
In the beautiful and extensive collection of British Land and Fresh-water 
Molluscs, in the Cabinet of Mr. John Pickering, are numerous forms of this genus, 
and I have applied to that gentleman for his assistance upon the recent as well as 
fossil Pisidia, and as he has devoted many years to the examination of these animals, 
I conceive his opinion to be of much more value than my own; he says (in Lit.) “1 
am of opinion, we have not more than five species of Pzsdia in this country, viz., 
P. amnicum, P. Henslowianum, P. pulchellum, P. obtusale, and P. pusillum. After 
selecting the first three species, there are many forms remaining, from which without 
much difficulty may be selected pust//um which appears a less variable species than 
most of the others; then follows od¢usale the most variable of the whole, differing in 
almost every locality, in some comparatively large and free growing, in others small 
