SCALPELLUM. 31 



barely) distinguishable into an upper and lower portioh ; the whole being nearly straight, 

 or very slightly concave. Apex extremely produced, narrow, and horn-like; curled 

 towards the carina; apparently (for the apex is broken) a considerable portion was 

 thickened, and must have projected freely. Occludent margin slightly arched, about equal 

 in length to the scutal margin, which latter in the lowest part is curved and projects a 

 little. Basal angle bluntly pointed. A rounded ridge (with a mere trace of a furrow on 

 its carinal side), almost disappearing in the lower part of the valve, runs from the apex 

 to the basal angle in a slightly curved course, strictly parallel to the carinal margin. The 

 rim of the valve along the occludent margin is rounded and strongly protuberant, and, 

 parallel to it, the surface is considerably depressed. Length of valve, when perfect, 

 1"2 of an inch. This variety differs from the first in the much greater straightness of the 

 carinal margin, in the occludent rim being more protuberant, and in the scutal margin 

 not being quite straight. One specimen presented a decidedly intermediate form, though 

 rather nearer to the first than to the second variety. 



Tergum. Var. III. Tab. II, fig. 7. The valves of this variety, of which I have seen 

 five specimens, were for a long time quite unintelligible to me, and I at first even thought 

 that perhaps they were rostral latera, but I now find that in outline, though not in 

 general appearance, owing to their great thickness, they closely resemble the terga of S. 

 magnum. One of the four specimens is almost exactly intermediate between the variety 

 last named and that now to be described ; hence there can be no doubt that they are really 

 terga. The chief characteristic of the valves of this variety is their narrowness, and the 

 solidity of their upper ends, which, together with a point of structure presently to be men- 

 tioned, makes me think it likely that they belonged to the individuals which possessed a 

 carina, hereafter to be described under the name of S. maximum, var. cglindraceum. Valve 

 smooth but with the lines of growth plain, extremely narrow, almost crescent-shaped ; 

 carinal margin considerably more concave than in Var. II, with a barely perceptible promi- 

 nence in the upper part, marking the commencement of the freely projecting portion, 

 and probably the point of upward extension of the carina. The occludent margin is 

 arched, and is equal in length to the straight scutal margin. From the apex there runs a 

 fine furrow (instead of a ridge and furrow, as in Vars. I and II,) to the basal angle, nearly 

 parallel to the carinal margin, but almost blending with it in the lower part of the valve. 

 The upper freely projecting portion is much thickened, and rendered almost horn-like, but 

 to a variable extent ; owing to this the width of the valve in the upper part also varies. 

 In the specimens most characteristic of the present variety, the rim of the valve along the 

 occludent margin is not at all, or barely, protuberant, nor is there any plain depression 

 parallel to the occludent margin : in the intermediate specimen, however, above alluded to, 

 the rim is protuberant and there is a plain depression, though both much less conspicuous 

 than in the tergum of Var. II. On the internal surface of the upper freely projecting 

 part, (marked with lines of growth,) there can be observed in two specimens a slight 

 and variable longitudinal depression ; judging from what occurs in the recent genus 



