139 
The moveable tergum (Pl. XI, fig. 2) is rather broad and quadrangular. The axial 
ridge, which is to be considered as the fifth ridge, is beautifully curved, grows wider from the apex 
towards its inferior extremity and projects freely at the basi-scutal corner of the valve. The part of 
the valve between the axial ridge and the curved longer occludent margin is flat, has a triangular 
shape and shows distinct growth-ridges, which run parallel to the perfectly straight basal margin. 
The part of the valve between the axial ridge and the nearly straight shorter occludent margin 
has a triangular shape also, but its surface is not flat but divided into ridges separated by more 
or less distinct furrows. Of these ridges the first runs along the shorter occludent margin of the 
valve: it is narrow, slightly shorter than the second and can only be made out when the valve 
is isolated. The second is broader, especially towards its free extremity, which projects as a 
tooth on the scutal margin of the valve. The third is shorter than the second and the fourth, 
but, like the other ridges, it grows broader towards the scutal margin of the valve. The fourth 
is the broadest of all, it is distinctly curved and its surface is convex; its extremity projects 
slightly — not so strongly as the second and fifth ridges — on the scutal margin of the valve. 
The scutal margin shows altogether only three distinct excrescences or teeth, the extremities of 
the first and third articular ridges not projecting at all. 
The rostrum (Pl. XI, fig. 3) is large, bulky, of a quadrangular shape. It is strongly 
convex, especially in its inferior part; it has the basal margin distinctly curved, the upper 
margin hollowed out. A rim along the upper margin is divided into two or three little teeth, 
which serve for the articulation of the scutum with this valve, and also to support the extremity 
of the big upper tooth of the carina. The carinal margin is furnished with five teeth (when 
fullgrown), four in the younger specimens; these teeth diminish in size from the upper towards 
the basal margin of the valve. Near the carinal margin the surface of the valve is distinctly 
undulated, the crests of the waves being only feebly prominent however, and not distinctly 
ringed. In other specimens more distinct prominent ridges could be made out, their direction 
being from the carinal margin to the scuto-lateral angle of the valve. 
The carina (Pl. XI, fig. 4 and 4a) is parallelogram-shaped as far as the part of the 
valve situated at the side of the moveable valves is concerned; a smaller triangular part of the 
valve forms a sharp angle with the main part and bends over to the side of the fixed scutum 
and tergum. The upper margin of the main part is convex, its lateral margin nearly straight, 
its basal margin more or less irregular according to the convex surface of the object to which 
the shell is usually attached. The rostral margin has the same number of teeth as the carinal 
margin of the rostrum, their size corresponding with the excavations of the other valve. The 
undulations of the surface are, as a rule, slightly better developed than on the rostrum. The 
triangular part which bends over to the side of the fixed tergum shows distinct growth-ridges, 
and the upper margin rests against the basal margin of the carinal-lateral portion of the fixed tergum. 
The fixed tergum (Pl. XI, fig. 5) has the ordinary shape: a well developed middle 
portion of triangular shape and two triangular lateral portions. One of these is situated at the carinal 
side of the valve: it has a rather long and convex occludent margin and a shorter and exca- 
vated basal margin; it describes a distinct angle with the middle portion, to which it is attached 
by a rather long and concave margin. The remaining lateral portion, which might be called the 
x 
