36 CATALOGUE OF 



What I would regard as the type form is represented by those 

 figured from Addington in Surrey (PI. XIII, f. 15 ; XV, f. 15) 

 or Gateshead in Durham (PI. XIII, f. 16; XV, f. 7) and again 

 from Wniana in Roumania (PI. XIII, f. 8 & 31 ; XVI, f. 15) and 

 Naples (PI. XIII, f. 32 ; XV, f. 13). In the Sebeto at the last- 

 named locality an extremely oval form is met with (PI. XIII, f. 7 

 & 30; XVI, f. 16). This is closely paralleled by specimens from 

 the Pleistocene at Uxbridge. The normal tumidity of the species 

 is shown in PI. XIII, f. 1 & 4, whilst f . 2 & 3 are inflated examples 

 and f. 5 & 6 instances of greatest compression. 



There is one well marked form, a lake or still-water form, which 

 almost amounts to a variety. In this the shell is rounder than the 

 type, and more compressed (PI. XIII, f. 5, 17, 23, 25 & 26) whilst 

 the hinge being narrower and lighter is less arcuate and the flexure 

 less pronounced. The cardinals, though still retaining their dis- 

 tinctive characters, are, owing to the reduced width of the hinge-plate, 

 forced into a position more or less parallel therewith (PI. XV, 

 f. 5); the lateral teeth on the other hand, beyond being some- 

 what lighter, remain as in the type (PI. XVI, f. 3-6 & 12). 



It is among the fossil examples especially from the Pleistocene of 

 the Thames Valley, where individuals that lived under varying 

 conditions have been swept together by the great river, that the 

 greatest variation iu shape is met with, and interesting series can 

 be picked out (PI. XIV, f. 4, 5, 7 & 8 ; XVII, f. 1-4). Some spe- 

 cimens are almost orbicular, whilst others are subtrigonal in outline 

 (PL XVII, f. 3 t). The latter are curiously paralleled by examples 

 sent me by Herr Lynge from Lyngby-Bugsvoer So, Seeland (PL XV, 

 f. 20 e, g}. Herr Lynge, however, was not able to point to any 

 notable peculiarity of the environment that could have led to the 

 production of this extreme form. 



The most triangular examples met with come from the Pleistocene 

 at Grays (PI. XVIII, f. 12 i, /.-, I), where at times they are hard to 

 distinguish on account of the thick* tied dentition from the associated 

 P. svplnvrn. The nearest living individuals resembling them are 

 some from Devonshire (PL XV, f. 16 ; XVI, f. 2 a, b). 



Individuals showing a more or less complete exchange of hinge 

 formula for the two valves are nomctimes met with, as in other 

 species of the genus. The most striking example is that from 

 New town ards, Co. Down (PL XVI, f. 7 & 8), in which the right 

 valve has a single anterior and doulle cardinal teeth, whilst the left 

 valve has the lateral teeth doubled. Among fossil forms two right 

 valves from the Pleistocene of Swanscombe (PL XVII, f.'3q& *) 

 have each a single anterior lateral, and one (PL XVI] I, f. 7) has 

 both laterals single, whilst a left valve (PL XVII, f. 4p) has a 

 double anterior lateral. In an abnormal right valve from the 

 Holocene of Elie, Fifeshire (PL XVIII, f. 6) there is a single in lieu 

 of a double posterior lateral. 



Specimens marked " P. cinereum, British " are in the Hanley 

 collection (B.M. 1907. 12. 30 : 524-5) and may have furnished the 

 figure in Forbes and Hanley (63, pi. xxxvi, f. 2). 



