102 CATALOGUE OF 



ridges rounded, umbonal sloping gradually, distal 

 steeply. 



L.Y. a. n. about ^ the length of the hinge-line, very stout, straight 

 (but occasionally strongly deflected inwards) ; base very 

 deep, strong and swollen ; apex distal side of centre, 

 very prominent, rounded obtu?e, distally directed ; 

 ridges somewhat rounded, distal sloping at first very 

 steeply, then gradually, umbonal steeply. 

 2. prominent, triangular, base continuous with that of 

 n. //., apex sharp-pointed, directed outwards and back- 

 Avards towards the umbo. 



4. lamelliform, sharp, traversing nearly the whole width 

 of the hinge-plate diagonally from the umbo, parallel 

 to the posterior side of 2. 



p. n. about 3 the length of the hinge-line, stout, straight, or 

 with slight inward curvature; base very deep, strong 

 and swollen ; apex distal side of centre, very pro- 

 minent, rounded acuminate, distally directed ; ridges 

 rounded, umbonal long, gradually sloping down to a 

 lower level than the distal, distal at first nearly 

 vertical, then gently sloping. 



Dimensions. Schmidt's measurements translated into millimetres 

 arc : Long. 4'5, Alt. 3'9, Crass. 3'3. Baudon gives 5 to 6 x 5 to 

 6 x 3 to 3'5. The largest recent British specimens are from the 

 Thames at Eton 4'2x4 - 8x2'7 and Budworth Mere (Cheshire) 

 4-2 x 3-5 x 3'4, whilst a single valve from the Pleistocene at Grays 

 is 4-6 x 4-6 x 2. 



On the whole this species is very distinct, the strong, character- 

 istic hinge and trigonal form, even when the appendiculae are not 

 developed, sharply distinguishing it from its congeners ever since 

 Croinerian times (PL XXV, f. 14; XXVI, f . 9 & 10) down to the 

 present day (PI. XXV, f. 11 ; XXVI, f. 1). 



Sometimes, especially in the young stages, it assumes a more 

 rounded form (PL XXV, f. 10 6, 15 ; XXVI, f. 2 b ; XXVII, f. 1). 



Occasionally it is simulated by other species, especially in the 

 fossil state. This is notably the case in the Pleistocene of Crayford- 

 Erith, where, as already mentioned (p. 95), a very heavy, somewhat 

 triangular variant of P. Tienslowanum is very abundant (PL XXV. 

 f. 13 ; XXVI, f. 13). At this locality P. supinum is comparatively 

 rare, whilst at Grays it is the most abundant species. 



There are also instances in which thickened variants of P. caser- 

 trinum, especially the right valves (cf. PL XVIII, f. 12 k & 1), closely 

 approach P. supinum. 



