LOWER HELDERBERG ROCKS. 191 



8tropIiodoiita 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, biftu'cating, 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 Strophomma 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 strise; 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 sufficiently strong, but the character is fully shown in fig. 1 i.) 



This species is very closely allied to Strophomma {Lepfmna) sowerhyi of Barrande 

 ( Sil. Brach. aus Bcehmen, PI. 21, f. 1 & 2 o, 6, c, e); but differs from those figures 

 in b*,'ing 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 individnal 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. 



