LOWER HELDERBERG ROCKS. 17 S 



The striae are distinctly tubular, with openings at the more con- 

 spicuous lines of growth. The fine concentric striae are often scarcely 

 visible in the silicified specimens. 



This shell has much the general habit of a Carboniferous species usually referred 

 to 0. umhraculum ; but it is not resupinate, and dilfers materially in other characters. 

 The specimen figured on Plate xv is much distorted, perhaps in part by accident. 

 A more symmetrical form is figured on Plate x a. 



I have thus far seen no young of this species; and among very extensive col- 

 lections continued in the Helderberg for more than ten years, only two or three 

 specimens have been found, and these appear to be of full-grown individuals. 



Fig. 3 a. View of dorsal valve and area of tlie ventral valve. 



Fig. 3 b. Ventral valve of the same specimen, showing the contracted and distorted bealc. 



PLATE X A. 

 Fig. 13 a. Dorsal view, showing the extent of area, the broken beak, etc. 

 Fig. 13 6. Ventral valve of the same, showing the abrupt contraction of the striae towards 



the beak, which is broken off below the point of attachment. 

 Fig. 14. Enlargement of striae, showing the tubular openings at the imbricating lines of 



growth. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains. 



Orthis assiinilis ( n. s.)- 



Plate XV. Fio. 1 a - e. 



SHEUi suborbicular, sometimes a little longer than wide : valves nearly 

 equally convex above the middle. Dorsal valve most convex in the 

 middle, and sloping to the front and sides. Ventral valve most convex 

 towards the beak, depressed and broadly sinuate below : beak promi- 

 nent, acute, incurved, and extending beyond the opposite valve. Area 

 longer than half the width of the shell ; the height equal to one-third 

 the length. Foramen large. 



SuRFACK finely and somewhat evenly striated. Vascular impressions of the 

 ventral valve foliate, occupying a broad ovate space, limited on the 

 sides by a continuation of the* brachial lamellae. 



