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the moveable valves, a smaller part, also of triangular shape, is bent over to the side of 
the fixed compartments. The larger part is strongly developed longitudinally, its elevation 
surpassing considerably that of the carina. It might again be considered as composed of 
two parts: one between the basal margin of the valve and the crest of the angle formed 
with the part on the other side of the shell, and one between the carinal margin and the upper 
or scutal margin of the valve. These two parts are separated from each another by means of 
a stronger ridge, which runs almost longitudinally. Along this ridge two other less prominent 
ridges may be seen, each terminating in a slightly prominent tooth, serving for the articulation 
of this valve with the carina. A fourth articular ridge runs along the scutal or upper margin 
of the valve and forms at its basal extremity, i.e. at the scuto-carinal angle, a larger tooth, 
fitting in an excavation of the rostral margin of the carina. The surface of this valve has distinct 
growth-ridges and is, moreover, irregularly folded, the folds not reaching to the apex of 
the valve. 
The carina is broad or wide, not high. Its surface is distinctly bowed longitudinally 
as well as transversely, its shape being in consequence elongately spoon-like. Its upper or 
tergal margin is short and concave, its basal margin is much longer and convex. The basal 
part of the valve is somewhat curved over the rounded-spine to which the shell is attached and 
thus a small part of its margin is seen when looking at the shell from the side of the fixed 
scutum and tergum. The lateral margin of the valve is short, and articulates with the triangular 
part of the fixed tergum, which bends over to the front of the valve. The rostral margin of 
this valve is long and shows four excavations in which fit the extremities of the four articular 
ridges of the rostrum. The widest of these excavations is that for the reception of the strong 
tooth of the rostrum at its scuto-carinal angle. The part of the valve along the rostral margin 
develops into a kind of articular ridge and running parallel to it are two other ridges, which, 
however, can best be seen at their extremities where they project as teeth, which fit between 
the teeth at the extremity of the articular ridges of the rostrum. The carina shows also growth- 
ridges, and the part of the surface near the lateral margin is, moreover, distinctly folded, the 
direction of the folds changing from longitudinal, near the lateral margin, to nearly transverse 
parallel to the growth ridges. 
The fixed scutum (PI. XI, fig. 12) is considerably larger than the fixed tergum. It 
is more than one and a half times as broad as high. Its basal margin is long, slightly convex 
and shows numerous shallow excisions corresponding to .vertical folds at the outer surface of the 
valve. The tergal margin is short and shows at the outer side a narrow radius, overlapping 
the fixed scutum at its scutal margin. On the inner surface there is, near the distinctly 
beaked upper angle, an excavated part bordered by a ridge and serving for the reception of 
the upper angle of the fixed tergum. The occludent margin is concave and relatively long, 
the lateral margin is composed of a short upper part, which lies along the upper part of the 
scutal margin of the rostrum, and a longer under part along the lateral margin of the rostrum. 
The latter part of the margin has an irregular, sinuous course. The valve itself is divided 
into three parts: a large triangular part between the basal margin and a somewhat curved 
rim running from the basi-rostral angle to the apex; a triangular part along the occludent 
15 
