112 MOLLUSCA FROM THE CRAG. 



4. PISIDIUM PUSILLUM, Turton. 



TELLINA POSILLA, Turt. Conch. Diet., p. 167, 1819. 



CYCLAS PUSILLA. Turt. Brit. Biv., p. 251, t. 11, figs. 16, 17, 1822. 



Id. Land and F. -Water Shells, pi. 1, fig. 7. 

 GIBBA. Alder. Trans. Nat. Hist. Soc., Newcastle, vol. i, p. 41. 

 FONTINALIS. Drop. Moll. Terr, et Fluv. France, p. 130, pi. 10, fig. 8, 1805. 

 EUGLESIA HENSLOWIANA. Leach, MSS., fide Jenyns. 

 PISIDIUM PUSILLUM. Jenyns. Trans. Camb. Phil. Soc., vol. iv, p. 302, t. 20, figs. 4 6, 



1831. 



Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 123, t. 37, fig. 10, and 

 pi. 0, fig. 9, 1849. 



Spec. Char. Testa pusittd, orbiculato-ovatd, compressiusculd, subincequilaterd, subti- 

 lissime striatd ; umbonibus prominulis. 



Shell small and slender, roundedly ovate, somewhat compressed, slightly inequi- 

 lateral, very finely striated ; umbones but little projecting. 



Length, |-th of an inch. 



Locality. Clacton, Copford (/. 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 

 believe, 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.) " I 

 am of opinion, we have not more than five species of Pisidia 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 pusillum which appears a less variable species than 

 most of the others ; then follows obtusale the most variable of the whole, differing in 

 almost every locality, in some comparatively large and free growing, in others small 



