SCALPELLUM. 19 
the largest scutum, tergum, carina and upper latera in Mr. Wood’s collection, they are 
very nearly double the size of the same valves in the largest specimen of S. vulgare 
seen by me, namely from near Naples, which had a capitulum eight tenths of an inch 
in length; and they are more than double the size of the same valves in any British 
specimen. Scalpellum magnum probably had a capitulum one inch and a half in length. 
Carina (Tab. I, fig. 14 and f) abruptly, almost rectangularly bent, with the umbo of 
srowth seated just above the bend, at about one third or one fourth of the entire length of 
the valve from the upper point ; form linear, with the lower part slightly wider than the upper. 
Exteriorly the surface is rounded with no central ridge, excepting near the umbo, where the 
narrowness of the whole valve gives it 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 8. 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-parietes meeting each other, the valve being thus 
added to at its upper end. The carina of 8. 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 S. vu/gare: 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. 
Scuta (fig. 1 e) much elongated, trapezoidal, slightly convex ; umbo placed on the occlu- 
dent margin at about one fourth of the entire length of the valve from the apex, so that the 
valve grows upwards and downwards. Occludent margin straight, slightly hollowed out 
above the umbo, forming rather less than a right angle with the basal margin, which latter 
is at right angles to the lateral margin. The tergal margin is separated from the lateral by 
a slight projection (beneath which the margin is a little hollowed out), and from this pro- 
jection there runs a ridge, often very conspicuous, to the umbo. The part above the ridge, 
stands at rather a lower level than that below it, and the lines of growth on it are generally 
less distinct. This is connected with the fact, as ascertained in S. vulgare, that the valve, 
during its earliest stage, grows only downwards, the ridge thus indicating the original form 
of the valve and tendency of the lines of growth. On comparing that part of the scuta 
beneath the umbo and ridge, in the present species (Tab. I, fig. 1 c), with the whole valve 
in some other species, for imstance in S. fossula (fig. 4a), in which the umbo is seated at 
the apex, as it was in the first commencement of growth in S. vulgare and magnum, it 
