LOWER HELDERBERG ROCKS. 
175 
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 shelf has much the general habit of a Carboniferous species usually referred 
to 0. umbraculum; but it is not resupinate, and differs 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 the ventral valve. 
Fig. 8 b. Ventral valve of the same specimen, showing the contracted and distorted beak. 
PLATE X A. 
Fig. 13 a. Dorsal view, showing the extent of area, the broken beak, etc. 
Fig. 13 b. 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. 
Ortliis assimilis (n. s.). 
Plate XV. Fig. 1 a - e. 
Shell 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. 
Surface 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. 
