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 form a 

 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. |. H. Torbay. 



The young (P. LAMELLATA. Turt B.Z.t. I./ 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. TUBERCULATUS. Turton, B. 5. t. 1. /. 7. 8. 



jp. 547. Open at the posterior end, which is subros- 

 cated ; with a rough tubercle on the margin above the 

 teeth, and a single oval calcareous plate at the hinge 

 extending to the posterior extremity : posterior area 

 with rough ribs which disappear anteriorly. |....li. 

 Torbay. 



