SPECIES OF riSlDJUM. {$5 



3. prominent, arcuate, flat-topped; a thin, sharp, fairly 

 parallel to shell-margin ; b thickened, strongly sulcate, 

 curving diagonally across the hinge-plate almost to its 

 inner margin. 



p. i. about g the length of the hinge-line, strong, straight, 

 compressed ; base strong, straight ; apex near the distal 

 end, not very prominent, pointed obtuse ; ridyrg fairly 

 sharp, sloping away fairly equally and steeply, the 

 urabonal descending a triflp lower than the distal. 

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

 narrow, eiect; apex distal side of c^ntre, prominent, 

 acuminate ; rulgeg compressed, umbQnal sloping I airly 

 gradually, distal steeply. 



L.V. a. n. about j\ the length of the hinge-line, narrow, straight ; 

 base strong, not much swollen ; apex central, very 

 prominent, acuminate ; ridges fairly sharp, distal veiy 

 steep at first, then sloping gradually, umbonal steep, 

 descending to a lower level than the distal. 

 2. very prominent, sharply triangular, its base parallel 

 with the hinge-plate arid continuous with that of a. //., 

 the apex curving outwards and twisted spirally (some- 

 times considerably) points towards the posterior dorsal 

 margin. 



4- lamelliform, sharp, curving Diagonally back at a high 

 angle across the hinge-plate to .almost its inner margin. 

 p. ii. about ^ the length of the hinge-linje, narrow, strong, 

 curving with the shell-margin ; byse strong and straight ; 

 apex near distal end, very prominent, distally directed, 

 acuminate ; ri<>ges almost sharp, umbonal descending 

 fairly steeply, distal very steep at first, then gradually 

 sloping. 



Dimensions. Two of the finest examples seen are in the collec- 

 tion of Mr. C. Oldham. One frojn Fitton Hall, Oldham (Lanes.), 

 measures : Long. 7, Alt. 6, Crass. 4 mm. ; the other from Marsworth 

 Iteservoir (Herts.) is 6'5 x 5*5 x 4 mm. 



In external conformation this is a most variable species and may 

 at times, especially wl.en dwarfed, resemble forms of P. pvsillwn 

 (e. (j. PI. XIII, f. 25 ; XVI, f. 12), P. personatum (e. cj. PI. XIH, 

 f. 196; XVI, f. Qa&b) and even P. nitidum (e. y. PI XIV, 

 f. 7 6 & d). 



Its leading characters are its usually larger size as compared with 

 its congeners, and the fact that its greatest anterior projection lies 

 rather below a lateral line taken through the adductor scars. The 

 sharp anterior flexure of the hinge, especially in the right valve, 

 with the disposition of the lateral teelh instantly distinguish it, as 

 does the position of the cardinals (2 <&' 4} i" the left valve which 

 are placed well athwart the hinge-plate (PI. XV, f, 3). In 

 P. personatum and P. nitidum these teeth are on the other hand 

 fairly parallel with the hinge-line. 



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