VARIETIES OF OYSTERS. 163 



existing : colour reddish, yellowish, brown, or white, or of 

 intermediate shades, variegated by straight or diverging 

 streaks or blotches of some of those tints : margins rounded 

 in front and at both sides, and notched or indented by the 

 impression of the ribs ; in the young the upper edge of the 

 angle on the right-hand side, which lies under the large 

 ear, has a row of curved spines, which are arranged like 

 the teeth of a saw : beaks prominent : ears of unequal size, 

 especially in the young, that on the left-hand side of the 

 upper valve and on the right of the lower valve being the 

 largest ; all of them are sculptured like the rest of the 

 shell, the ribs diverging from each side of the beak out- 

 wards ; the right hand ear of the lower valve is notched 

 at the base, and it is smaller than the opposite one on the 

 left hand of the upper valve, in order to make an opening 

 and passage for the byssus : hinge-line straight : cartilage 

 short but strong : ligament long and slender : hinge-plate 

 strengthened by a thick and knob-like rib on each side of 

 the beak, to form the sides of the cartilage-pit : inside 

 pearly, microscopically pitted, and sornetimes very finely 

 and closely striate lengthwise : muscular scars slight. 

 Length i'65, breadth 1*45. 



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

 rush (Portlock and A. Bell). E. Cape of Good Hope 

 (Dunker) ! Every rocky coast from Shetland to Cornwall, 

 often on oyster-beds, and attached in the adult state by the 

 whole or last-formed part of its lower valve to the inside of 

 old bivalve shells, or to rocks, Eschara foliacea, and other 

 substances. The depth of water in which it lives varies 

 from 5 to 85 fathoms, and the young are occasionally 

 found at low water mark on some shores where the tide 



retires for two or three fathoms. In a fossil state P. pusio 



F 2 



