PISIDIUM. 27 



inconspicuous : inside nacreous-white : hinge-line nearly 

 straight ; cardinal teeth very minute and almost impercep- 

 tible ; lateral teeth not well developed, except towards their 

 outer edges, which are strong and sharp : muscular andjj)«7//o7 

 scars scarcely visible. L. 0*1. B. 0*15. 



Habitat : Marshes, ponds, ditches, and stagnant water 

 from Zetland to Guernsey. It also occurs in Silesia, 

 Sweden, and France ; and I have found it in Prussia. 

 It has probably escaped notice in other parts of the Con- 

 tinent. 



This species differs from all its congeners in its oblong 

 or rhomboid shape, which is principally owing to the 

 greater extension of the posterior side, and to the beaks 

 being consequently placed so much out of the centre, as 

 well as to the compression and nearly straight outline of 

 the lower or front margin. It is considerably more 

 ventricose than P. nitidum, which it resembles in its 

 gloss and sculpture. The umbonal strise are, besides, 

 not perceptible in the species under consideration ; and 

 the tube does not appear to have the margin of its 

 orifice plaited. The body has usually a rosy or reddish 

 hue in the upper part, which is discernible in the dried 

 animal. 



It was not without much hesitation that I adopted the 

 name given by Scholtz for this species, because in a 

 Supplement to the second edition of his work he con- 

 sidered it to be a variety of P. fontinale ; but the colour 

 of the animal, which at first induced him to propose 

 this as a distinct species, appears to form a good and 

 constant mark of distinction, and one of the epithets 

 which he applied to the shell (" langlichrundlich ") is 

 very appropriate. The only other species of Pisidium 

 besides this, which Scholtz has noticed, are fontinale, 

 amnicum, and obtusale. Whether it may ultimately be 

 united with nitidum is, however, a question which I, for 



c 2 



