6 TESTACEOUS MOLIAWUSCA. 
fold above the hinge without cells: a smooth tubercle on the inner 
margin above the teeth : a single accessorial valve at the hinge. 7. . 15. 
P. Siuicuta. Lam. 5.—Del. 1.7.19. Narrow-oblong not beaked, 
white, with radiating spinous ribs, and concentric transverse strie ; 
a callous elongated tooth in each valve. 1.—Mauritius. 
P. Cosrata. Lin. 1111.—D. p. 36.—Lam. 6.—E. t. 169. f. 
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 smallerend. 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 striee which posteriorly are muricated with small erect 
scales not arched beneath, an anterior space devoid of the striz: 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. $..14%5--V. America. 
P. Crucicer. 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. =i, ..145.—C. America. 
P. Campecuensis. Gmel. 3216.—CanpiDA, var. D. p. 37.— 
List. t. 432. f. 275.—CuiLoensis, 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. 34.—S. America. 
P. Cuausus, Gray in Bowdich. Transversely elongated, ante- 
riorly acuminated, posteriorly rounded ; tumid, with rather distant, 
concentric, raised strize, (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? 1}... 2¢--Africa. 
P. Susrruncata, Sow. Z. P. 1834.—An. P. Siminis, Gray 
in Yates, N. Zeal. Ovate oblong, scabrous, anteriorly with a 
