SPECIKS OF PISIDIUJT. 95 



towards the distal end, fairly prominent, acuminate; 

 ri(f(/es sharp, umbonal descending gradually, distal 

 very steeply at first, then very gradually, 



Dimensions. Slioppard's measurements give, when translated 

 into millimetres: Long. 5, Alt. 4 mm. Jenyns gives the same, 

 but add*, Crass. 3-5 mm. A specimen from the lied Bridge 

 Pond, Hampstead Heath (London), is .5-1x4x3-4, and one in 

 Mr. Oldham's collection from Baguley (Cheshire) 6x5x4, but 

 those are surpassed by one received from Herr Lynge from 

 Lynsy-Bagsvcer 8<">, Sealand, which attains 6'6x58x4'6 mm. 

 (PI. XXIV, f. 5). 



In very young shells the appendiculre of course appear well 

 back on the valve (PL XXIJI, f. 25 29 c, <l) and only with growth 

 become relatively apical. Not infrequently these appendicula 1 are 

 wanting, when the shell closely resemliles P. subtruncatitm : ihe 

 best examples are those from Preston (Lanes.) [B.^J. 1913.1.1 : 

 1-9J. It may, however, be distinguished by the sharper umbones, 

 by the fact that the cardinal 3 of the right valve is more curved 

 and its posterior limb, />, more sulcate ; whilst the cardinals 2 & 4 

 of the left valve are diagonal rather than parallel to the hinge-line, 

 and 2 is sharply pointed instead of rounded. 



An interesting form is met with in the Pleistocene deposits of 

 Grays and Crayford-Erith in which the shell is more trigonal in 

 shape and much thicker and heavier in the hinge than the typical 

 form (PI. XXV, f. 13 & XXVI, f. 13). At first sight, indeed, it 

 sceaas difficult to separate it from the associated .P. supinum. Its 

 anterior lateral teeth, however, are proportionately longer and a. /. 

 is not so inwardly directed, nor is the cardinal 3 so sharply flexed ; 

 whilst in the left valve the apex of the cardinal 2 points more 

 backward and 4 curves further forward over the apex of 2. The 

 P. supinum, moreover, is more truncate posteriorly, the more 

 sharply triangular cardinal 2 points directly outwards, and its 

 edges show traces of folding back, and 4 does not reach forwards 

 over the apex of 2, whilst the fissure between 2 & 4 crosses the 

 hinge-plate at a higher angle, ifore normal forms from Crayford- 

 Erith are shown on Pis. XXIII, f. 29 & XXIV, f. 8 & 9. In the 

 Crayford-Erith beds P. liensloivunum is by far the commoner, 

 P. sup'nium being comparatively scarce. At Grays, on the other 

 hand, the latter is the abundant form, outnumbering the former by 

 about. 4 to 1 . Except in the instance just described, the species 

 shows no variation beyond the average from Cromerian to recent 

 days. The single valve from the Coralline Crag is too immature to 

 admit of comparison. Specimens from Hampstead (PL XXIII, 

 f. 27) show the normal amount of inflation, those from Baguley 

 (PL XXIII, f. 28) arc the most swollen. The former may also be 

 taken as examples of the average form (PL XXIV, f. 7). Those 

 from the Thames (PL XXIV, f. 2) show the strengthening of the 

 hinge consequent on their having to contend with strongly flowing 



