139 



The moveable ter gum (PI. 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 (PI. 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 (PI. 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 (PI. 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 



