96 Annals of the South African Museum. 



at the umbo, and may exceed the total length. The valve is almost 

 vertically truncated anteriorly and in its anterior part attains such 

 relatively great depth that a flattened frontal face of considerable 

 extent becomes developed. 



In the youthful stage the ribs of the flank are concentric in 

 arrangement, extending from the carinal angle to the frontal border. 

 As growth proceeds, subsequently formed ribs terminate anteriorly 

 at the frontal border but posteriorly at an increasing distance from 

 the carinal angle. The ribs at the same time attain a more steeply 

 inclined direction, and the last-formed rib of the frontal series is 

 very short and almost vertical. In the early adult stage narrow, 

 closely spaced ribs extend between the carinal angle and the upper 

 terminations of the swollen frontal ribs, or may be looked upon as 

 the attenuated upward prolongations of these since the two sets are 

 so far in continuity. Successively formed narrow ribs, constituting 

 a posterior series, have their upward terminations slightly removed 

 from the carinal angle, and in the posterior half of a fully grown 

 specimen these crowded ribs of the posterior series entirely occupy 

 the flank. The ribs of the anterior series are very prominent and 

 coarsely tubercular, and may be elevated to a height of 7 or 8 mm. 

 above the surface of the interspaces. These ribs are very widely 

 spaced, the interspaces reaching 8 mm. in breadth in the adult stage ; 

 on the frontal face the ribs sweep round in a broad curve towards 

 the anterior valve-margin. The development of the earlier-formed 

 ribs is not continued in the later growth-stages, so that in a large 

 specimen the frontal face is partly without sculpture in its upper 

 part, towards the valve-border. The ribs of the posterior series are 

 at first vertically directed, but the majority of them have a marked 

 backward inclination when traced downwards. They are relatively 

 weak, and are separated by interspaces less than 2 mm. in breadth. 

 The whole surface of the flank is crossed by crowded lines of growth 

 which are most plainly visible in the wide interspaces between ribs 

 of the anterior series. 



The carinal angle is represented by a blunt, rounded fold in the 

 adult shell. The area is very narrow, and is without ornamentation 

 in the adult stage, except a strongly impressed median longitudinal 

 groove. The escutcheon is relatively broad and is deeply excavated 

 in form. 



Dimensions. 



Length (approximate) 75 mm. 



Height measured from the umbonal region 82 ,, 



Greatest depth of a single valve 45 ,, 



