235 



ON THE RECENT MISAPPLICATION OF THE NAMES 



PISIDIUM NITIDUM AND PISIDIUM PUSILLUM 



OF JENYNS. 



By a. W. STELFOX, M.R.I.A. 



(Read before the Society, October 13th, 1917). 



The number of species of Pisiduim recognised by Gwyn Jeffreys in 

 his "British Conchology" was only five, and at the time I received 

 my first lessons in malacology Jeffreys's views were accepted generally, 

 but in the light of our present-day knowledge of the genus it is not 

 surprising that uncertainty was often felt in referring specimens defi- 

 nitely to one or other of the four smaller species. No difficulty, it is 

 true, arose with typical examples of such characteristic species as 

 P. heiisIowanu?n, P. pulchellmn (regarded by Jeffreys as varieties of 

 his P. fontinale) or P. milium^ and it seemed to me that P. nitidum 

 was equally unmistakable, for its clean and shining appearance, and 

 to quote Jenyns's own words " the peculiar striae drawn with great 

 regularity across the umbones near the apex of each valve, and cut 

 rather more deeply than the rest " distinguished it at once from the 

 shells with dull epidermis, usually encrusted with extraneous matter, 

 that one regarded as P. pusillum. Indeed, the common practice was 

 to refer any small Pisidium in which the umbones were more or less 

 central to P. tiitiduvi if the shell were clean, and to P. pusilhtm if it 

 were encrusted, whilst if the umbones were nearer to the posterior 

 end P. fontinale was available as a dumping-ground for it. 



When, after years of comfortable acquiescence in this method of 

 determining the species, I came to regard the subject from the eco- 

 logical standpoint, doubt as to its efficacy soon arose. I found that 

 the two forms commonly regarded as P. pusillum, type, and P. pusil- 

 lum var. grandis, nearly always occurred in association, but showed 

 marked differences at all stages of growth, and the possibility of their 

 being distinct and not merely varieties of a single species was forced 

 upon me. Shells of these associated forms which I sent some time 

 afterwards to Mr. B. B. Woodward were referred by him to P. per- 

 sonatuni Malm and P. casertanum Poli, and my preconceived idea of 

 their distinctness was confirmed ; but it did not then occur to me to 

 ask, if P. pusillum be P. personatum, and P. pusillum var. grandis be 

 P. casertanum, what then can be the shell that Mr. Woodward regards 

 as P. pusillum. It certainly did not occur to me that it was the shell 

 I had always looked upon as Jenyns's P. nilidum. 



I have said already that the identification of P. nitidum in the old 

 days did not seem difficult, and this may have been because my idea 

 of the species was based upon shells with a shining epidermis and 



