SCALPELLUM. 41 



three or four times as wide as the ridges themselves ; the hues of growth are very fine and 

 narrow. 



Carina (fig. 7, a, h, g) ; narrow, considerably arched : tectum flatly arched, obscurely 

 subcarinated : parietcs rectangularly inflected, somewhat concave, and not longiiudlnall;/ 

 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, g. 



Scutum (fig. 7,/) ; moderately convex, with the apex acuminated : lateral margin nearly 

 parallel to the slightly arched occludeut 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 w^hen the valve is viewed from above, is destitute of the longitudinal 

 ridges. 



In 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 carinal 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. The valve is slightly depressed, parallel to the occludent 

 margin. A small portion of the apex projected freely ; internally, in the upper part, 

 rather nearer to the occludent than to the caiinal 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 striae 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 



