54 CATAI.OutTK OF 



p. r. about J- the length of the hinge-lino, stout, strong, 

 slightly curving inwards ; base strong and swollen ; 

 apex toward the distal end, prominent, rounded obtuse: 

 ridyes rounded, umhonal descending fairly steeply, 

 distal very steeply. 



p. in. about i 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 ; 

 ridjes rounded and sloping fairly equally and gradually. 

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

 pit an irregular shaped, more or less prominent, shelly 

 mass or callus, is almost always present. 



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

 strong, straight ; base very strong and swollen ; apex 

 central, exceedingly prominent, rounded acuminate ; 

 ridyes fairly sharp, sloping about equally and very 

 steeply, the umbonal descending to a lower level than 

 the distal. 



2. very prominent, obtusely triangular, apex directed 

 backwards towards the umbo, base continuous with 

 that of a. n. 



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

 tion immediately above 2, close to and parallel with 

 the 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. 71. next the ligament-pit. 



Dimensions. From 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. pusittum 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 

 si/.e and having a rounder outline, all the teeth are more outwardly 

 directed ; in the right/ valve the anterior lateral a. in. 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. pusiUum it differs in having a heavier hinge, in which 

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



