G TESTACEOUS MOLLUSCA. 



fold above the hinge without cells : a smooth tubercle on the inner 

 margin above the teeth : a single accessorial valve at the hinge. f . . 1 \. 

 P. Silicula. Lam. 5. — Del. l.jf. 19. Narrow-oblong not beaked, 

 white, with radiating spinous ribs, and concentric transverse striae ; 

 a callous elongated tooth in each valve. 1 . — Mauritius. 



P. Costata. Lin. 1111. — Z>. p. 36.— Lam. 6.—E. t. 169./. 

 1, 2.— Reeve, t. 23.— Sow. G.— W. t. 2. f. 4. Oblong-ovate, 

 whitish or cream-coloured, covered with strong crenulated longi- 

 tudinal ribs, which become abruptly closer at the smaller end. 2 . . 5. 

 — S. Europe and America. 



P. Truncata. Say, Jour. A. N. S. Phil 2. p. 321. Oblong, 

 sub-pentangular, posteriorly beaked and obtusely cuneiform in the 

 middle, anteriorly broadly truncated at the tip : with transverse and 

 longitudinal striae which posteriorly are muricated with small erect 

 scales not arched beneath, an anterior space devoid of the striae : hinge 

 callus formed of the duplicature of the hinge margin, with a small 

 toothlike callosity on the inner edge projecting backwards : tooth 

 curved, prominent, slender, flat, f . . !•£§ — N. America. 



P. Crtjciger. Sow. Z. P. 1834. Elongated-oblong, anteriorly 

 rounded, posteriorly with the ventral edge open but not forming a 

 beak : with very close and numerous narrow radiating ribs covering 

 the whole surface except the anterior slope, where the very close 

 and concentric wrinkles which roughen the ribs become almost 

 lamellar : hinge margin reflected, its inner edge with an oblique 

 tooth-like callus : accessory valve solitary, anterior, and trans- 

 verse. T "L .. 1 Jq. — C. America. 



P. Campechensis. Gmel. 3216. — Candida, var. D. p. 37. — 

 List. t. 432. f. 275. — Chtloensts, King in Z. J . vol. 5. Elon- 

 gated, smooth and rounded anteriorly ; posteriorly not beaked, with 

 radiating rows of rather distant spinous scales situated on the con- 

 centric elevated lines of growth, (which are not at all undulated) thus 

 forming rhomboids ; hinge-plate cellular, little elevated ; accessory 

 valves 3, the two posterior and sub-triangular are angulated and 

 channelled posteriorly, and truncated anteriorly, where they are 

 terminated by an upright triangular piece : apophysis deeply chan- 

 nelled, not broad. 3|-. — S. America. 



P. Clausus, Gray in Bowdich. Transversely elongated, ante- 

 riorly acuminated, posteriorly rounded ; tumid, with rather distant, 

 concentric, raised striae, (except at the anterior end) whose distant 

 but regular scales form seeming rays from the hinge ; no posterior 

 gape, the area being filled by a smooth or radiatingly ribbed space : 

 hinge margin much elevated, and abruptly truncated at the umbones 

 anteriorly : hinge without an apophysis, two tubercular teeth on each 

 hinge margin : accessory plate ? \} . . 2§- — Africa. 



P. Subtruncata, Sow. Z. P. 1834. — An. P. Simtlis, Gray 

 in Yates, N. Zeal. Ovate oblong, scabrous, anteriorly with a 



