[19]

W O R K S OF d a r w i n : v o l u m e 1 4 Exteriorly the surface is rounded with no central ridge, excepting near the umbo, where the narrowness of the whole valve gives its a carinated appearance; basal margin rounded. From the umbo two faint ridges run to each corner of the basal margin, separating the steeply inclined parietes from the roof - a character of some importance in the cretaceous species of this genus: outside of these two ridges there are other two ridges, not extending down to the basal margin, and separating the parietes from the intra-parietes, which latter being united at their upper ends, and produced upwards, form that part of the carina which is above the umbo. By comparing the lateral views of the carina of the cretaceous S. fossula (fig. 4c), and of this species, it will be seen, that the apparently great difference of the umbo of growth being either at the apex, or, as in this species, sub­ central, simply results from the lines of growth of the intra-parieties meeting each other, the valve being thus added to at its upper end. The carina of S. magnum, examined internally, is found often to be narrower under the umbo than either above or below it, a character I have not seen in the recent S. vulgare. The lateral width or depth of the valve (measured from the umbo to the inner edge) is also greater than in 5 . vulgare: this portion is internally filled up and solidified. No part of the apex of the valve projected freely. The longest perfect specimen which I have seen, is half an inch in length; but I have noticed fragments indicating even a greater size.