SCALPELLUM. Al 
three or four times as wide as the ridges themselves ; the lines of growth are very fine and 
nalrow. 
Carina (fig. 7, a, 6, g); narrow, considerably arched: tectum flatly arched, obscurely 
subcarinated: parietes rectangularly inflected, somewhat concave, and not longitudinally 
ridged, like the tectum, about two thirds as wide as half the tectum: basal margin bluntly 
pointed, the two edges meeting each other at rather above a right-angle; a trace of a 
rounded ridge separates the tectum and parietes; in the upper part of the carina there 
is no trace of intra-parietes, therefore the section of the upper half of the carina is only 
four-sided, see fig. 7, 9. 
Scutum (fig. 7, f) ; moderately convex, with the apex acuminated : lateral margin nearly 
parallel to the slightly arched occludent margin, and at right angles to the straight 
basal margin ; a distinct ridge runs from the apex to the baso-lateral angle, which is 
distinctly prominent and rather sharp. The valve, above a line running from the apex 
to the tergo-lateral angle, is inflected; and the narrow portion thus inflected, which 
cannot be seen when the valve is viewed from above, is destitute of the longitudinal 
ridges. : 
Tn a specimen from the Grey Chalk of Dover, in which the internal surface was visible, 
there was, above the well-marked depression for the adductor muscle, a prominent, central, 
slightly oblique ridge, with the inner occludent edge of the valve widened and slightly 
hollowed out on the one side, and with a trace of a furrow on the other or tergal side. 
Terga (fig. 7, c,d); flat, oval, with the scutal angle rather protuberant ; basal angle not 
sharply pointed, from it to the apex there runs an obscure furrow, which furrow in the lower 
part of the valve is central, but higher up is situated at about one third of the width of 
the widest part of the valve from the carmal margin; in the lower part of the valve, the 
lines of growth (and consequently the margins of the valve) make with this furrow, equal 
angles on its opposite sides. he valve is slightly depressed, parallel to the occludent 
margin. A small portion of the apex projected freely; ¢xternally, in the upper part, 
rather nearer to the occludent than to the carinal margin, the valve is prominent, and 
this part is marked with two or three little ridges (c) ending abruptly downwards. 
Size of largest specimen,—length of carina, °85 of an inch; of scutum, from the 
apex to the basal margin, rather above ‘6; of terga, °55. I do not, however, know 
that these valves belonged to the same individual. 
Variety. In the British Museum there is a scutum, and in Mr. Flower’s collection 
there is a tergum, both from the Grey Chalk of Dover, which are most closely allied to, 
if not identical with, the above valve. The raised striz on both are rather further apart 
and are less prominent. In all the other characters the scutum is identical. The tergum 
differs in its carinal margin, being rather more angularly bent, and in there being no furrow 
running from the apex to the basal angle; but these differences are trifling and insufficient 
for distinguishing a species. Amongst some specimens most kindly sent me by Roemer, 
there is a tergum from the Planer of Sarstedt (Chalk-marl), which is identical with this. 
f 
