132 
Rostrum and carina are of nearly the same shape and size. From the middle of the 
front of the shell, where these two valves meet, they bend over to the sides of the same, where 
they overlap the margins of the fixed scutum and tergum with triangular portions growing broader 
towards the basis of the shell. Both valves have the upper margin of the front part perfectly 
straight and the umbo at the extremity of that margin; their basal margin is arched, each 
equalling about one fourth the circumference of a circle. From the upper margin where the nearly 
horizontal moveable valves are attached to the basis, the whole shell increases considerably in 
width and this is the case especially with that part of the wall which is formed by the rostrum 
and the carina. The strongly curved surface of both valves shows numerous irregular ridges 
of growth. The suture with which the two valves articulate is not straight, but shows nume- 
rous teeth growing smaller from the upper margin towards the basis. These teeth form the 
extremities of as many oblique ridges (interlocking folds) running from the apex of the valve 
to the rostral margin of the carina or to the carinal margin of the rostrum. The number of 
these ridges seems to increase with age: I counted from 6—8 of them in different specimens. 
The fixed scutum and tergum form the remaining part of the wall. They do not 
increase so much in width from the upper margin downwards, and their position is much steeper 
in consequence. The fixed scutum is larger than the fixed tergum; it consists of a triangular 
middle part, the tip of which nearly touches the apex of the moveable scutum, and the basal 
margin of which is rather long and distinctly curved; of a broad triangular lateral part, the 
upper margin of which as occludent margin forms part of the edge of the shells orifice and the 
basis of which does not quite reach the basis of the shell. This lateral triangular portion is 
overlapped by the lateral margin of the rostrum, forming here a kind of a radius. The third 
part of the fixed scutum is narrow, it increases slightly in breadth from above downwards and 
overlaps the scutal margin of the fixed tergum. The structure of the latter valve is nearly the 
same: it also has a triangular middle part, the tip of which is opposed against the tip of the 
moveable tergum and two triangular lateral parts, one side of which as occludent margin forms 
part of the edge of the orifice of the shell. The triangular portion situated at the side of the 
fixed scutum is overlapped by the narrow radius-like part of that valve, that at the side of the 
carina is also overlapped by a triangular kind of a radius forming the lateral part of the carina. 
The surface of these valves is smooth and irregularly folded ; distinct growth ridges are visible on the 
surface of the triangular parts, the upper margins of which form the occludent margins of these valves. 
The form of the basis is irregularly circular, the transverse diameter (that which runs 
parallel to the basal margin of the moveable valves) is slightly longer than the diameter which 
is vertical to this direction. 
The size of this species is relatively large: it seems to be one of the largest of the 
genus. In the specimen figured the dimensions are: 
height about 10 mm. 
greatest diameter of the basis 14.5 mm. 
distance from the apex of the rostrum to that of the carina 10 mm. 
In the largest specimen collected the corresponding measurements are: II, 16.5 
and 13 mm. 
