SPECIKS OF PISIDIUM. 121 



(i. ni. about \ the length of a. ;., very obscure, closely 

 apposed to the shell-margin. 



3. very proininen 4 , faintly arcuate, thin, sharp, very 

 slightly thickened posteriorly, flat-topped, sides vertical. 

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

 straight, erect ; bate strong and swollen (especially in 

 old specimens) ; apev central, distally directed, promi- 

 nent, acuminate to rounded obtuse ; ridyes slightly 

 rounded, sloping about equally, fairly steeply (in old 

 specimens more gradually). 



p. ni. not quite g the length of p. /., to which it is parallel, 

 erect, narrow, but spreading umbonally, and becoming 

 confluent with p. /. ; apex central, scarcely prominent, 

 very obtuse-pointed ; ridyes compressed, sloping about 

 equally. 



L.V. a. if. about y\ the length of the hinge-line, strong, narrow, 

 erect, straight; hrtse very strong and swollen; ape.c 

 central, or on umbonal side of centre, very prominent, 

 acuminate; ridyes fairly sharp, distal exceedingly steep 

 at first, then almost horizontal, umbonal descending 

 very steeply to a slightly lower level than the distal. 



. very narrow and prominent, with vertical sides, erect, 

 the rounded cusp bent sharply outwards. 



4- narrow, thin, sharp, slightly inclined backwards, 



diagonally across the hin^e-plate. 



p. ii. about ,L the length of the hinge-line, narrow, strong, 

 straight, with slight outward tilt; bast strong and 

 swollen (especially in old specimens) ; apex distal side 

 of centre, very prominent, distally directed, acuminate; 

 ridyes somewhat rounded, umbonal descending fairly 

 gradually, distal nearly veitical at first, then sloping 

 very gradually. 



Dimensions. Lamarck simply says: " Largcur, pres de 4 milli- 

 metres " ; Pfeiffer's measurements converted into millimetres are, 

 Long. 2, Alt. 2*5, Crass. l - o mm.; Jcnyns', similarly treated, are 

 3*5x3x3 mm. One from Lytham (Lanes.) collected by Mr. W. 

 M. Webb is 3'7 X 3'3 x 3-4 (PI* XXX, f. 1. 5), whilst it. Air. Cooper's 

 collection one from Criccieth (Carnarvon) is 3-"5 x 3'5 x 2*5 mm. 



This species more commonly retains the hairy periostracnm to 

 the adult state. Its less tumid forms may be distinguished from 

 the rounded individuals of other species by the shortness of the 

 hinge-line, the peculiar manner in which it is cut away at the 

 iimbo, by the position of the apices of the lateral teeth which are 

 at most central, and often on the umbonal side of the centre, a 

 feature shared by no other species, by the almost total suppression 

 of the anterior lateral a. ///., and by the narrow, straight-sided, 

 boldly upstanding cardinals, especially L 2 whose rounded cusp is 

 bent sharply over at the top as to impart to the whole tooth the 

 appearance of a staple. 



