Grenus.—DISCINA. 
Inequvalve, rounded ovate, slightly depressed ; valves 
of equal size and each marked with a central or- 
bicular dise which in the upper valve is undivided 
and submammillary in the centre, in the other valve 
is extremely white and divided by a transverse 
jissure. 
D. OstreorpEs. Lam.—F. p. 376.—ORBICULA 
Norvecica. (not Lam.) Sow. Inn. T. 13. p. 468. 
t. 26. f.2.—Sow, G.f. 3,4,5.— Reeve. ¢. 126. f. 3,4.5. 
—Upper valve brown with fine longitudinal ribs crossed 
by concentric wrinkles; margin more or less waved ; 
inside under the apex with two irregular callous ridges ; 
under valve white, concentrically wrinkled, a disc 
round the fissure to which the peduncle adheres., 
0,40...3. 4 Very doubtfnl native species; on ballast. 
Grenus.—CRANIA. 
Inequivalve, suborbicular ; lower valve almost flat, 
pierced internally with three unequal oblique per- 
forations ; upper valve very convex, strengthened 
internally with two prominent callosities. 
C. Rostrata. Hening. Mon. Cran. p. 3. f. 3.— 
Desh. in Lam. 7. p. 302.—C. PErsonata. Sow. Lin. 
T.13. ¢.26. f. 3.—Sow. G. f. 1,2.—Reeve. ¢. 128. 
f.2.—Patetta Distorta. Mont. Lin. T. 11. €. 13. 
