[i6]

B . parietibus et basi, sed non radiisporisperforatis; testá albo cum roseo aut obscurè purpureo longitudinaliter pictâ, interdum puré albâ. S cuto longitudinaliter

tenuiter striato: interné, adductoris cristâ admodum aut modicè prominente. Parietes and basis, but not the radii, permeated by pores; shell longitudinally striped with white and pink, or dull purple; sometimes wholly white; scutum finely striated longitudinally; internally, adductor

ridge very or moderately prominent. Fossil in coralline crag (Ramsholt and Sudbourne), rarely in the red crag (Sutton);

This species has caused me much trouble. It will be convenient first to make a few remarks on the recent specimens; I examined several from Panama and California, which, though differing greatly in colour, resembled each other in their scuta having the adductor ridge extremely prominent, and in having (PI. I, fig. 4n) an almost tubular cavity for the attachment o f the lateral depressor muscle - characters which at first

[17/8] W O R K S OF d a r w i n : v o l u m e 1 4 appeared of high specific value; but I soon found other specimens from Panama in which these peculiarities were barely developed. I then examined a single specimen from the Philippine Archipelago, resemb­ ling in external appearance one of the Panama varieties, but differing in the scuta being externally strongly denticulated in lines instead o f being merely striated-in the adductor ridge being far less prominent - and in the spur of the tergum being broader and more truncated; I therefore considered this as a distinct species. I then examined a single white rugged specimen from the coast of Peru, which differed from the Philippine specimen in the shape of the well-defined denticulations on the scuta, and in some other trifling respects, and in the segments of the posterior cirri bearing a greater number of spines; with considerable doubt, I also named this as distinct. But when I came to / examine a large series of fossil specimens from the coralline crag of England, and others from northern Italy, from Portugal, and from the southern United States, I at once discovered that the form of the denticuli on the striae of the scuta was quite a worthless character - that in young specimens the scuta were simply striated - that the prominence o f the adductor scutorum ridge and the depth of the cavity for the lateral depressor muscle varied much (as in the case of the recent specimens), owing apparently to the varying thickness of the valve-that in the terga the spur varied considerably in length and breadth, the latter character being in part determined by the varying extent to which the edges of the longitudinal furrow are folded in - and lastly, that in young specimens the basal end o f the spur is much more abruptly truncated than in the old. Hence I was led to throw the three recent forms, originally considered by me as specifically distinct, into one species; but I may repeat that this considerable variation in the prominence of the adductor ridge, and in the depth of the pit for the lateral depressor muscle - the pit in some cases becoming even tubular - is a very unusual circumstance.