131 
varying in form and thickness, and more particularly in the 
shape and size of the lunule and curvature of the umbones. 
Perhaps they had better be regarded as varieties of one species. 
Geological position and locality,—the Clypeus Grit of 
Rodborough Hill. Rare. 
Lucrna crypzata. PI. IV, fig. 8. 
Shell convex, suborbicular, sub-equilateral, lunule rather 
large, umbones small, mesial, hinge margin slightly curved, 
base rounded, concentric lines closely arranged, regular, inter- 
rupted by numerous folds of growth, which become rugose near 
the base. Anterior side produced. Height 12 lines, diameter 
13 lines, diameter through both valves 6 lines. 
Geological position and locality,—Clypeus Grit, Rodborough 
Hill. 
_ Sowerspya ELoneaTA. PI. IV, figs. 11, lla. 
Shell thick, smooth, shining, transversely lengthened, equi- 
lateral, convex, with a posteal subacute, oblique angle, and 
narrow flattened space posterior to it; anterior side produced, 
its lower extremity curved elliptically, posterior lower ex- 
tremity angulated; umbones small, little elevated, surface 
ornamented with regular longitudinal striations, moderately 
separated, and passing over the posteal oblique angle and 
the surface posterior to it. There are two or three folds of 
growth, and much variability in the degree of convexity and in 
the obliquity of the posterior angle. Length 15 lines, height 
9 lines. 
Geological position and localitity,—Clypeus Grit of Rod- 
borough Hill. 
Prcten Crypeatus. Pl. 5, fig. 1. 
Shell small, convex, sub-equilateral, with radiating coste 
(about 60) closely arranged, slightly curved, but obsolete on 
the upper part of the shell; the whole surface concentrically 
striated, auricles ribbed, apex slightly curved. 
