VARIETIES OF OYSTERS. 1 71 



each side from about three-fourths of the distance from 

 the front margin ; slope below the byssal sinus strongly 

 toothed or serrate : beaks prominent : ears nearly equal, 

 sculptured by ribs which radiate from the beak ; the ears 

 of the lower valve slightly project beyond and overlap 

 those of the upper valve ; byssal notch deep : hinge-line 

 straight : cartilage rather large : ligament narrow and slight : 

 hinge-plate somewhat broad, minutely striate across ; trans- 

 verse rib strong and raised on each side of the cartilage- 

 pit : inside fluted or grooved, so as to correspond with the 

 folds of the outside ribs ; each shoulder under the ear is 

 furnished with a thick ledge, which rests on that in the 

 opposite valve, thus giving additional support to the hinge 

 and preventing its being too closely pressed : muscular scars 

 distinct. Length 2*35, breadth 2*5. 



Var. i, lineata. Shell white, with a brown line run- 

 ning down the crest of each rib. P. lineatus, Da Costa, 

 Brit. Conch., p. 147, pi. 10, f. 8. 



Var. 2, tumida. Shell more swollen and deeper. 



Var. 3, elongata. Shell smaller, and longer than 

 broad. 



HABITAT: N. of Hebr., 530 f. (C. and T.) F. 

 England, Ireland, Scandinavia, and Italy. Common on 

 all sandy coasts, and gregarious, in 6-90 fathoms. Var. i. 

 Not infrequently found with specimens of the usual colour, 

 and sometimes having a mixed hue. Var. 2. Plvmouth 



O j 



(J. G. J.) ; Exmouth (Clark) ; Cork (Humphreys). Var. 

 3. Loch Torridon, Ross-shire (J. G. J.) ; Birterbuy Bay, 

 co. Galway (Bailee): rare. This species is a common 

 fossil in the Scotch glacial deposits, and in the Norwich, 

 Red, and Coralline Crag. Steenstrup informs me that he 

 has found it in Iceland ; Mohr has recorded it from the 



