LOWER HELDERBERG ROCKS. 187 



Stropliodoiitu cavnmbona ( n. ».)• 



Plate XXI. Fig. 1-3. 



Shell subsemicircular, from two- to four-fifths as long as wide, usually 

 contracted below the extremities of the hinge line. Cardinal border 

 sloping a little from the beaks, nearly o^quite equalling the greatest 

 width of the shell. Dorsal valve concave in the umbonial region and 

 near the hinge, very convex in the middle and towards the front. 

 Ventral valve flat or concave : beak very small, scarcely elevated 

 above the cardinal margin. Hinge line straight, crenulated. Area linear, 

 partly common to both valves, transversely striate on the ventral valve. 

 Foramen small, narrow, closed by a callosity. 



Surface marked by coarse irregular radiating strice, which increase by 

 implantation : concentric striae very fine and closely arranged. 



This species has the form of S. punctulifera, and possesses also many of the ex- 

 ternal characters. The best preserved specimen shows no puncta upon the exterior, 

 and the striae do not bifurcate as in characteristic specimens of that species. The 

 surface of older specimens is mai-ked by strong, elevated, angular stria;; often with 

 comparatively broad, intermediate, nearly flat spaces which are marked by fine and 

 almost imperceptible concentric striae, and at intervals by the commencement of 

 other stronger radiating striae, which gradually become more and more numerous 

 as well as more subdued towards the margin ; so that the surface of this part often 

 presents the appearance of being evenly striated. 



The striae are not always acutely angular, but rounded upon the upper and middle 

 portions of the shell, with spaces between scarcely exceeding the strise, and nu- 

 merously bifurcated below as in fig. 1 /, Plate xxi. In other specimens, and par- 

 ticularly the young, the stria; are prominently angular, with the spaces equal to 

 the striae. 



In young specimens the shell is nearly flat, or slightly curved towards the margin : 

 as it grows older, it becomes more convex on the dorsal and more concave on the 

 ventral valve, the surface characters meauAvhile assuming various modifications. 



Fig. 1 a, c. Ventral and dorsal valves of young specimens. 



Fig. 1 b, d. The striae in profile, and the surface enlarged. 



Fig. 1 e. A partial cast of the ventral valve. 



Fig. \ f. The dorsal valve partially exfoliated, and showing the crcnulations along the hinge 

 line : the stria; are rounded and frequently bifurcating. 



