86 CATALOGUE OF 



acuminate; ridyes sharp, distal fairly steep, umbonal 

 less so. 



3. prominent, lamclliform, almost straight, slightly diagonal 

 to the hinge-line. 



p. i. about 3 the length of the hinge-line, narrow, strong, 

 straight; base strong, straight; apex towards distal 

 side of centre, prominent, rounded-obtuse ; ridyes 

 somewhat sharp, at first fairly steep, the distal the 

 more so, then sloping away more gradually. 

 p. HI. about | the length of, and parallel to p. /., straight, 

 strong; apex near distal end, prominent, rounded- 

 obtuse ; ridyes sharp, umboual descending fairly gradu- 

 ally, distal very steeply. 



L.V. a. II. nearly | the length of the hinge-line, narrow, strong, 

 straight ; base strong and swollen ; apex central, or 

 towards umbonal side of centre, very prominent, acu- 

 minate, slightly distally directed ; ridyes somewhat 

 sharp, distal descending at first very steeply, then 

 almost horizontal, umbonal' very steep and descending 

 to lower than the distal. 



2. prominent, bluntly triangular, erect, parallel with 

 margin of hinge-plate ; base continuous with that of 

 a. 11. 



4. lamelliform, sharp, flat-topped, anterior portion parallel 

 with 2, posterior portion curving towards inner hinge- 

 margin. 



p. II. about g the length of the hinge-line, narrow, strong, 

 straight, slightly outwardly directed ; base strong ; apex 

 distal side of centre, distally directed, prominent, 

 acuminate ; ridyes sharp, umbonal descending fairly 

 straightly and gradually, distal very steep, especially 

 at first. 



Dimensions. Malm's measurements are : Long. 3'25, Alt. 2-6, 

 Crass. 2 mm. An example from 15aguley (Cheshire) in Mr. C. Old- 

 ham's collection is 4 x 3 x 2-25 and one of his from Cemmaes 

 (Anglesey) attains 4'25 x 3*5 x 3 mm. 



Its most characteristic features are : externally it is very in- 

 equilateral, in this respect resembling P. hensloivanum, from 

 inappendiculate examples of which it may be distinguished by its 

 rounder umbones, as well as the details of its hinge, for the cardinals 

 are practically parallel with the hinge-line and 2 & 4 with each 

 other and 2 is round-pointed and directed outwards, whilst in 

 P. lunslowanum 2 is sharp-pointed and backwardly directed and all 

 the cardinals are placed diagonally in respect to the hinge-plate. 

 These same features of the cardinals and the fact that the apices 

 of the laterals are further from the umbones than in P. nitidum dis- 

 tinguish even its rounded forms, from elongate individuals of the 

 last named. 



The species does not appear to have changed its characteristics 

 from Cromerian times to the present day. Normally it is only 



