30 
posterior side inflated, closed, with oblique concentric 
wavy subdenticulated ridges dorsally, the ventral area 
smooth ; anterior side open, truncated, (and continued 
when perfect in a thin coriaceous expanding cup,) with 
merely coarse concentric lines of growth ; the back with 
two small accessorial appendages, with the edges of the 
valves reflected at the posterior end, so as to forma 
kind of double obtuse keel, which reaches as far as the 
longitudinal groove, near the end of which is a kind of 
raised joint where the accessorial valves are fixed ; 
apophysis rather short and flat; an erect somewhat 
triangular and rather concave plate on the hinge mar- 
gin. %. 12. Torbay. 
The young (P. Lametzata. Turt B.5.t. 1. f.5, 
6.) being destitute of the posterior ventral area, gapes, 
and is rounded and not truncated anteriorly. It is de- 
void also of the accessorial valves. 
P. Tusercutatvus. Turton, B.5.t. 1. fi 7. 8.— 
F..547,—Open at the posterior end, which is subres- 
cated ; with a rough tubercle on the margin above the 
teeth, and asingle oval calcareous plate at the hinge 
extending to the posterior extremity: posterior area 
with rough ribs which disappear anteriorly. 4....12. 
Torbay. 
