LOWER HELDERBERG ROCKS. 
1©H 
Stropliodonta beckii (n.s.)* 
Plate XXII. Fig. 1 a - t. 
Shell semielleptical or subquadrate : length sometimes equal to the 
width (though usually from two-thirds to three-fourths as great). Ven¬ 
tral valve very depressed convex : beak very small. Dorsal valve flat 
or a little concave near the hinge, slightly convex near the front. Hinge 
line crenulated, generally equal to the greatest width of the shell, but 
sometimes less. Area linear, confined to the ventral valve. Foramen 
small, linear, usually closed. 
Surface marked with strong, regular, closely arranged, bifurcating, ra¬ 
diating striae, crossed by fine obscure concentric lines, and more or less 
regular concentric wrinkles which curve outwards on approaching the 
hinge. 
Interior of shells striato-punctate, the hinge line crenulated almost 
to its extremities. The muscular area of the ventral valve flabelliform, 
more or less strongly defined at its margins. 
This is one of the most beautiful as well as most remarkable species of this genus 
in the rocks of the Helderberg group. Although corrugated like Strophomena rugosa , 
it is unlike that shell, in being almost entirely flat, and without the geniculation 
of that species. 
In its young state, this shell is marked by regular, round, bifurcating striae; the 
concentric wrinkles being scarcely developed. As the shell grows older these undu¬ 
lations become strong corrugations, which at first distinctly mark the upper part of 
the shell, and finally cover the entire surface. (In the figures 1 d - e, these cor¬ 
rugations are not sufliciently strong, but the character is fully shown in fig. 1 i.) 
This species is very closely allied, to Strophomena ( Leptcena ) of Barrande 
( Sil. Brach. aus Bcelnnen, PI. 21, f. 1 & 2 a, 6, c, e); but differs from those figures 
in being more coarsely striate, with the concentric wrinkles stronger, especially 
near the beak : they are, also, generally more curved outwards near the hinge. 
The lateral margins of our shell are likewise, in most specimens, more contracted 
below the extremities of the hinge, than in the examples given by Barrande. 
Fig. 1 a. A young individual in which no corrugations are developed. 
Fig. 1 b, c. Two specimens exhibiting the extremes of form in the extension of the hinge 
line, and showing the beginning of the development of the corrugations. 
