27 
PLAGIOSTOMA rigida. 
TAB. CXIV.—Fig. 1. 
Srrc. Cuar. Gibbous, obliquely obovate, with 
many diverging sharp thread-like ribs, and 
very minute intervenisg transverse strie; an- 
terior side long, straight, very concave; ears 
nearly equal. 
es RR ee 
Tur ribs are sharp, irregularly waved, a little zigzag, 
about three or four times their thickness distant from each 
other; the beaks are rather prominent, the shell entire at 
the edge, thin and rigid to the touch, in consequence of the 
sharpness of the ribs; except the greater depth, its proportions 
are the same as in P. punctata. The striae upon the space 
between the ribs are too fine to be seen without a lense. 
I picked this up in one of the quarries at Shotover, other- 
wise I have only met with it once, when my friend, Mr. 
Sheffield, lent me a specimen marked Oxford. P. ovalis 
is like it, except in being a narrower formed shell, and 
from the strie of growth the present does not appear to 
have been narrower when younger. 
PLAGIOSTOMA ovalis. 
TAB. CXIV.—Fig. 3. 
Spec. Cuar. Rather gibbous, oblique, elongated, 
oval, with many small ribs and minute inter- 
vening transverse stria; anterior side concave, 
slightly recurved. 
een eee 
Very nearly resembling P. rigida, so that at first sight 
it might be taken for the young of it, but it is of a nar- 
rower oval shape, the ribs are rounded, and at a distance 
from each other only equal to once their thickness; the 
length exceeds the width, taken at right angles to it. 
