SCALPELLUM. 31 
barely) distinguishable into an upper and lower portion; 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 carmal 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 carmal 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. 11. Tab. Il, 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 mtermediate 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. cylindraceum. Valve 
smooth but with the lines of growth plain, extremely narrow, almost crescent-shaped ; 
carinal margin considerably more concave than in Var. IJ, 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 carmal 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. IJ. 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 
