125 



GENUS. DISCINA. 



Inequivalve, rounded ovate, slightly depressed ; valves 

 of equal size and each marked with a central or- 

 bicular disc which in the upper valve is undivided 

 and submammillary in the centre, in the other valve 

 is extremely white and divided by a transverse 

 fissure. 

 D. OSTREOIDES. Lam. F. p. 376. ORBICULA 



NORVEGICA. (not Lam.) Sow. Lin. T. 13. p. 468. 



t. 26./.2. Sow, G.f. 3,4,5. Reeve. 1. 126./. 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... . A Very doubtful native species; on ballast. 



GENUS. CRANIA. 



Inequivalve t 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. H&ning. Mon. Cran. p. 3. f. 3. 



Desh. in Lam. l.p. 302. C. PERSONATA. Sow. Lin. 



T. 13. t. 26. /. 3. Sow. G. f. 1,2. Reeve, t. 128. 



/ 2. PATELLA DISTORTA. Mont. Lin. T. 11. t. 13. 



