LOWER PIELDERBERG ROCKS. 
fi§7 
Strophodonta cayumbona (n.s.). 
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 or 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 striae, 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 strise do not bifurcate as in characteristic specimens of that species. The 
surface of older specimens is marked by strong, elevated, angular striae* 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- 
nierously bifurcated below as in fig. 1 A, Plate xxi. In other specimens, and par¬ 
ticularly the young, the strise are prominently angular, with the spaces equal to 
the strise. 
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 meanwhile assuming various modifications. 
Fig. 1 a, c. Ventral and dorsal valves of young specimens. 
Fig. 1 6, d. The strise in profile, and the surface enlarged. 
Fig. 1 c. A partial cast of the ventral valve. 
Fig. If. The dorsal valve partially exfoliated, and showing the crenulations along the hinge 
line : the striae are rounded and frequently bifurcating. 
