nsiDiUM. 129 



mcr with the shape obhquoly oval, the profuncllty of 

 the valves not extentling to the margin, and the front 

 extremity rounded ; the latter with the outline subrhom- 

 boidal oblong and devoid of obliquity, the anterior end sub- 

 biangulated, and the ventricoseness so pervading the entire 

 shell that the lower margins almost form a rectangle at 

 their meeting : yet intervening examples certainly do occur, 

 knowing which we dare not subdivide pulchellum^ as was 

 once proposed by its author, who, in a recent communica- 

 tion to us, again reverts to his original opinion of the in- 

 tegrity of the species. One character, although varying in 

 intensity, is common to all the varieties, viz. the presence 

 of regular concentric stria3. 



Vak. a. Rather large, obliquely oval, ventricose, rather 

 deeply striated ; the umbones rather obtuse and 

 simple (Jenyns, pi. 21, f. 1). 



Var. j(3. Usually smaller and more finely striated ; the um- 

 bones rather acute (Jenyns, pi. 21, f, 2, S). 



Var. y. Obliquely oval, finely striated, compressed ; mar- 

 gins meeting at an acute angle. 



Var. ^. Subrhomboidal oblong, swollen almost equally 

 throughout, deeply striated ; margins meeting 

 at more than acute angles (Jenyns, pi. 21, 

 f. 4, 5). 



For the varieties a and y the name Jenynsil has been 

 proposed by Mr. Gray, in honour of the Eev. Leonard 

 Jenyns, who has so ably treated a genus which presented 

 such difficulties in its investigation, that after a long and 

 wearisome examination of its members, we have adopted 

 throughout (although with some little hesitation) the 

 species and varieties suggested in his valuable monograph. 

 In our eyes the variety ^ appears the least connected with 



