PLAGIOSTOMA rigida. 
TAB. CXI Y.—Fig. 1. 
Spec. Char. Gibbous, obliquely obovate, with 
many diverging sharp thread-like ribs,, and 
very minute intervening transverse strise; an- 
terior side long, straight, very concave ; ear* 
nearly equal . 
The 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 strife 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 striae of growth the present does not appear to 
have been narrower when younger. 
PLAGIOSTOMA ovalis. 
TAB. CXIY. — Fig. 3. 
Spec. Char. Bather gibbous, oblique, elongated* 
oval, with many small ribs and minute inter- 
vening transverse stride ; anterior side concave* 
slightly recurved. 
V ery 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. 
