SPECIES OF PISIDTCM. 45 



"P. testa orbiculato-ovnli, nitidissima, tenuiter striata; 

 urnbonibus obtusiusculis, striis paucis profundioribus. 



" Testa minim e variabilis, orbiculato-ovalis, parum insequi- 

 lateralis ; praacedenti \_P. pus'dlu,m~] paulo convexior, et pro 

 ratione longitudinis altior ; albo lutescens, nitidissima, raro 

 aut nunquam sorde aut rnbigine obtecta, tenuiter striata, 

 striis hie iliic, prsecipue 3-5 umbones transeuntibus, dis- 

 tmctius incisis : umbones obtusiusculi, dorsalem marginem 

 paulb superantes." 



The hinge-characters are as follows : 



Hlnfje (PL I, f. 9 ; III, f. 6) a little more than f the length 

 of the shell, strong, fairly wide, projecting well inwards, arcuate 

 with slight anterior flexure. 



R.V. a. /. about \ the length of the hinge-line, narrow, strong, 

 bowed and inclined inwards; base very strong and 

 swollen ; apex central, fairly prominent, pointed 

 obtuse ; ridyes fairly rounded, sloping fairly equally, 

 steeply at first, then giadually. 



a. in. about i the length of a. /., the whole forming a narrow, 

 arched prominence, somewhat apposed to the shell- 

 margin. 



3. strong, very prominent, crenulate top, sharply flexed ; 

 a narrow, approximately parallel to the shell-margin ; 

 6 thicker, strongly sulcate, diagonally directed across 

 the hinge-plate to near its inner margin. 

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

 curving somewhat with the hinge-margin, erect; 

 base strong, somewhat swollen ; apex distal side of 

 centre, not very prominent, obtuse-pointed ; ridges 

 sharp, sloping almost equally and fairly steeply. 

 p. in. about ^ the length of, and parallel to p. /., narrow, 

 erect ; apex central, prominent, obtuse-pointed ; ridyes 

 compressed, sloping about equally. 



L.V. a. n. about g the length of the hinge-line, narrow, strong, 

 nearly straight ; base strong and slightly swollen ; ape.v 

 central, very prominent, acuminate ; rid'jes fairly 

 sharp, distal at first exceedingly steep, then nearly 

 hori/.ontal, umbonal sloping very steeply and descending 

 to below the level of the distal. 



2. very prominent, obtusely triangular, fairly parallel 

 with the inner margin of the hinge-plate, base con- 

 tinuous with that of . //., pointed apex directed 

 slightly backwards. 



4 very prominent, thin, sharp, flat-topped, at first fairly 

 parallel to the shell-margin, then curving backwards 

 round 2, and almost reaching the inner margin of the 

 hinge-plate. 



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

 erect, straight ; base strong, scarcely swollen ; apex 



