6 CATALOGUE OF 



P. astartoides stand by themselves distinct from the rest. P. caser- 

 tanum, P. nitidum, P. personatum, and P. pittillttm, with probably 

 P. milium, form a group of transitional forms, that is linked on 

 the one hand through P. pulcliellum with P. subtruncatum, 

 P. henslowanum, P. supinum, and P. parvulum ; and on the other 

 hand, through P. steenbuchii with P. lilljeborgii, P. liibemicum, 

 and P. obtusale. P. vincentianum, with its depressed ligament-pit, 

 is a form apart. 



II. HlSTOKY OF THE BRITISH SPECIES. 



The history of the establishment of the several species here 

 recognized and the varying forms admitted by successive con- 

 chologists of note is briefly as follows : 



1774. 0. F. Miiller described (123), but did not figure, nor cite 

 any figure, his Tellina amnicum. Concerning the identity 

 of this species there fortunately is no doubt, although a 

 large synonymy now follows the name. 



1791. Gmelin described (97) a form under the name of Tellina 

 pusilla, referring to a figure by Schroter. The description 

 is inadequate for modern purposes : the figure conveys the 

 impression of a strongly striate form, such as P. pulchellum 

 or a young P. amnicum, and the outline rather favours the 

 latter. The name was used by Turton, Brown and others 

 for any or all of the smaller species of the genus, and since 

 it is impossible to say what Gmelin's shell was, the use of 

 the name must date from Jenyns, who first circumscribed 

 the species, as we know it. 



1795. Poli described (138) and figured Oardium casertamtm. 

 Neither figure nor description would enable one to identify 

 the species with certainty, but specimens sent me by Prof. 

 Bellini from the type locality, where it appears to be the 

 only species present, confirm the traditional belief on the 

 Continent that it is the same as Alder's P. cinereum. 

 1801. Draparnaud (51) created Cyclas fontinalis for certain forms 

 obviously belonging to Pisidiitm. His description is, how- 

 ever, too meagre, and his figures, published in 1805 (52, 

 pi. x.), too rough to enable one to say exactly what they were; 

 whilst, unfortunately, his specimens have been lost. If a 

 guess might be hazarded from the representations, it would 

 be that the type and var. y were P. casertanum, and the var. ft 

 one of the smaller species. The name has been largely 

 used on the Continent for forms identical with that taken 

 in these pages as P. casertanum (Poli). It would therefore 

 seem advisable to drop the name altogether. 



Draparnaud's only other species referable to Pisidium 

 was his G. palustris, a synonym for P. amnicum. 

 1818. Lamarck (92) separated out a small tumid form to which 

 he gave the name Cyclas obtusalis. The description was 



