SPECIES OF riSIDIUJf. 29 



rather narrow, erect, curving with the shell-margin ; 

 apex central, prominent, rounded obtuse ; ridyes some- 

 what rounded, descending about equally and fairly 

 gradually. 



L. V. n. //. nearly | the length of the hinge-line, stout, very strong, 

 straight; base very strong and swollen; apex central, 

 exceedingly prominent, pointed obtuse ; ridges fairly 

 sharp, descending about equally, at first steeply, then 

 very gradually. 



2. very prominent, triangular in the young shell, base 

 continuous with tr at of a. ///., apex pointing to the 

 umbo ; with growth the sides of the triangle fold back 

 more and more till they form a wedge-shaped pro- 

 minence closely resembling 3 b in the opposite valve ; 

 in very old shells the mark of the fold becomes 

 obliterated. 

 4. lamelliform, sharp, running diagonally backwards 



across the hinge-plate i'rom the umbo.. 



p. n. less than ^ the length of the hinge-line, narrow, strong, 

 curving with the shell-margin ; base strong, slightly 

 swollen; apex central, exceedingly prominent, rounded 

 obtuse; ridges sharp, descending about equally, fairly 

 steeply at first, then becoming almost horizontal.- 



The finest examples seem to come from the Cromerian at West 

 Him ton, one from the peat measuring Long. 9-4, Ait. 9, Crass. 

 ()(> mm., whilst one from the gravel is 9 x 9'3 X 5 mm. The speci- 

 mens from Grays rank next in size. 



The species may readily be distinguished from, its close ally 

 P. amnicum by its rounder outline and by the greater strength of the 

 external striae which stand up boldly especially on the very umbo 

 (PI. X, f. 7), whereas in P. amnicum they are by no means strongly 

 marked on the young shell even in the strongly striated examples 

 from Grays (cf. PI. V, f. 9 & 10). 



In the hinge of the right valve the paired lateral teeth are not 

 so parallel as in amnicum but curve towards each other enclosing a 

 very deep fossa, whilst in the left valve the cardinals 2 and 4 are 

 separated by a deeper fossa than in amnicum, nnd 2 is not so sharp 

 pointed. 



P. astartoides does not show any extreme variation in form as do 

 some of the other species of the genus. The earlier forms from the 

 Cromerian, both from the peat (PL X, f. 4) and from the gravel 

 (PI. XI, f. 1 ; XII, f. 8), are the rounder and the more strongly 

 sculptured. Specimens obtained at Swanscombe (PI. XI, f. 2 ; 

 XII, f. 1) come next ; but are more finely sculptured. At Grays 

 the form tends to become ovate-trigonal (PI. X, f . 3 ; XII, f. 2); 

 whilst at Crayford-Erith (PI. IX, f. 1 ; XII, f. 4) they are far 

 more oval, the extreme in this direction being met with at Clacton 

 (PI. X, f. 2 ; XII, f. 5). 



