54 CATALOGUE OF 



p. f. about I the length of the hinge-line, stout, strong, 

 slightly curving inwards ; base strong and swollen ; 

 apex toward the distal end, prominent, rounded obtuse ; 

 ridges rounded, umbonal descending fairly steeply, 

 distal very steeply. 



p. in. about l the length of p. /., stout, strong, lying close 

 to and curving with the shell-margin ; apex distal 

 side of centre, not very prominent, rounded obtuse ; 

 ridycs rounded and sloping fairly equally and gradually. 

 Callus : between the umbonal end of p. in. and the ligament 

 pit an irregular shaped, more or less prominent, shelly 

 mass or callus, is almost always present. 



L.Y. . //. about | the length of the hinge-line, fairly stout, 

 strong, straight ; base very strong and swollen ; apex 

 central, exceedingly prominent, rounded acuminate ; 

 ridges fairly sharp, sloping about equally and very 

 steeply, the umbonal descending to a lower level than 

 the distal. 



#. very prominent, obtusely triangular, apex directed 

 backwards towards the umbo, base continuous with 

 that of a. if. 



4. lamelliform, sharp, not very prominent, anterior por- 

 tion immediately above 2, close to and parallel with 

 ~t~he shell-margin, posterior portion curving to about 

 half-way across the hinge-plate. 



p. II. about \ the length of the hinge-line, fairly stout, 

 strong, curving with the shell-margin ; base very 

 strong ; apex on distal side of centre, very prominent, 

 rounded acuminate ; ridges fairly sharp, umbonal ridge 

 descending fairly steeply at first, then gradually. 

 Callus : a rounded swelling at the umbonal extremity of 

 p. JL next the ligament-pit. 



Dimensions. Prom Scotforth (Lanes.), coll. C. Oldham : Long. 

 4-75, Alt. 4, Crass. 3 mm. ; from Barnes (Surrey), 3-6 x 3'2 x 2-4 

 mm. 



Typical specimens are less inequilateral than most of the genus 

 and in this the species compares with P. pusillum and the smaller 

 individuals of the lake-form of P. casertanum. \Vhen clean the 

 umbonal regions have a dull silky, or satiny sheen. 



From its nearest ally P. nitidum it differs in attaining a larger 

 sixe and having a rounder outline, all the teeth are more outwardly 

 directed ; in the right valve the anterior lateral a. HI. is much 

 more closely apposed to the shell-margin, the cardinal 3 is nearly 

 straight instead of sharply flexed ; whilst in the left valve the 

 apex of the posterior lateral (p. //.) is nearer the umbo, and the 

 outer cardinal (4) is, save for its extreme posterior end, parallel 

 to the shell-margin. 



From P. pusillum it differs in having a heavier hinge, in which 

 the lateral teeth , ///. and p. in. are proportionately much shorter, 



