SPECIES OK riSIDlUM:. 61 



striae subtil issimae, non nisi oculo armato conspiciendae ; in 

 var. y nitida, striis distinctis, profundius iueisis ; umbones 

 subdepressi, parum prominuli, interdum subacuti." 



This fairly sums up the external characters. The particulars of 

 the hinge are as follows : 



Hinge (PI. I, f. 8 ; III, f. 4) about | the length of the shell, 

 narrow, especially so under the umbo, rather weak, strongly 

 curved. 



R.V. a. I- about -^ the length of the hinge-line, very narrow, 

 fairly strong, nearly straight ; base fairly strong, 

 slightly swollen ; apex near distal end, somewhat 

 distally directed, not very prominent, very obtuse- 

 pointed ; ridges sharp, distal descending first steeply, 

 then gradually, umbonal fairly gradually to a lower 

 level than the distal. 



a. in. about | the length of, and parallel to a. /., a deep 

 sulcus lying between the two, narrow, straight ; ape.r 

 about central, not very prominent, very obtuse-pointed ; 

 ridges sharp and sloping about equally and gradually. 

 8. laraelliform, sharp, not very prominent, faintly arcuate, 

 flat-topped, parallel with and close to the shell-margin, 

 posterior end (I) slightly thickened. 



p.i. about 3 the length of the hinge-line, narrow, fairly 

 strong, curving with the shell-margin, slightly outwardly 

 directed ; base strong ; apex near the distal end. not 

 very prominent, veiy obtuse and rounded; ridges 

 sharp, umbonal sloping gradually, distal somewhat 

 more steeply. 



f> in. about \ the length of, and parallel to p. /., a deep 

 sulcus dividing the two, narrow, sttaight; apex about 

 central, not very prominent, very obtuse and rounded, 

 slightly outwardly inclined; ridges sharp, sloping about 

 equally and gradually. 



L.V. a. ii. about \ the length of the shell, very narrow, strong, 

 straight; base strong and straight; apex near distal 

 end ; distally direct eel, very prominent, bluntly pointed ; 

 ridijes sharp, distal ridge descending very steeply at 

 first, then gradually, umbonal fairly steep, descending 

 to a lower level than the distal. 



2. lamelliform, rather sharp, prominent, parallel with 

 hinge-margin, top very flatly rounded, with faint trace 

 of backwardly directed apex, base continuous with that 

 of a. n. 



4- lamelliform, sharp, not very prominent, anterior por- 

 tion parallel to #, posterior curving slightly towards 

 inner hinge-margin. 



p. u. about \ the length of the hinge-line, very narrow, 

 fairly strong, curving with the shell-margin, slightly 



