112 
PLAGIOSTOMA lseviusculum. 
TAB. CCCLXXX1I. 
Spec. Char. Transversely obovate, oblique, 
convex, longitudinally ribbed ; ribs close, 
broad, irregular, very little elevated, con- 
vex ; anterior slope straight, hollow ; ears 
small, unequal. 
A smooth even formed shell, distinguished by the 
slight elevation of its ribs, if the radiating risings can be 
allowed the name of ribs, and the total absence of any 
striae between them ; the lines of growth are numerous, 
but very slightly marked ; the shell is thin so that the 
edge is undulated. 
Not uncommon in the Pisolite of Malton ; it is one of 
the many very distinct species so improperly confounded 
with P. rigidum by the Authors of a Geological Survey 
of the Yorkshire Coast, who have besides indulged them- 
selves in changing names, and several other liberties too 
palpably induced by error to merit particular notice. 
