132 
This shell is allied to P. Woodwardi, M. and L., and P. 
arcuatus, but differs from them in its outline and convexity; 
the absence of punctuations in the interstitial spaces further 
separates it from P. arcwatus. Height and diameter, half an inch. 
Geol. position and locality,—Clypeus Grit, Rodborough Hill. 
Oris Parner. PI. V, fig. 2. 
Shell subtrigonal, convex, inequilateral. Umbones elevated 
angular and curved; the anterior side crossed by four well 
defined plications, three of which occur at regular distances, 
the fourth is somewhat nearer the third and is near the lower 
border. Between each plication the surface has closely ar- 
ranged concentric lines. An elevated acute carina separates 
the anterior and posterior sides, parallel to which are slight 
depressions giving to the carina the appearance of a prominent 
thin ridge; the posterior side short, narrow; the anterior side 
much produced, Lunule very large, occupying the greater 
portion of the anterior border of the shell. 
The outline of the figure bears some resemblance to 0. 
Luciensis D’Orb, but the short and more curved carina, the 
oblique produced lower border of the anterior side, and the well 
marked plications across the surface distinguish it from that 
species. 
Height 8 lines, lateral diameter 7 lines. 
Geol. position and locality,—the Clypeus Grit of Rodborough 
Hill, where it is rare. 
Prerorerna Cryprata. PI. V, fig. 4, 4a, b, c. 
Shell small, sub-zquivalve, contrentrically marked with 
nearly obsolete lines of growth, crossed by faint longitudinal 
strie, umbones moderately large, the umbo of the left valve 
rising higher than the other, and above the hinge line; anterior 
wing produced, pointed; posterior wing rather small, trans- 
versely striated, its margin excavated by a longitudinal furrow, 
which becomes wider towards the lower extremity, and is 
longitudinally striated ; a slight ridge forms the inner border 
of the furrow; the right valve more flattened than the left, 
