174 OYSTERS, AND ALL ABOUT THEM. 



encircled by flake-white rings, and arranged in two or 

 three rows on each side ; the outermost row contains 

 some of the largest size, which curl at their tips and are 

 mostly of a yellow colour ; under this row are placed the 

 "eyes" or ocelli, about 50 on each side; these are of dif- 

 ferent sizes, not arranged symmetrically and black, with a 

 bright silvery pupil or nucleus in the centre. 



SHELL nearly circular and equilateral, except as in the 

 last species, thin, somewhat glossy : sculpture, mostly 7 

 rounded but. compressed ribs (the middle one being the 

 largest), which are much narrower than the interstices ; the 

 surface is covered with minute and close-set longitudinal 

 striae, which are crossed by equally fine but more remote 

 transverse plates, so as occasionally to form prickles at the 

 point of intersection ; the sides are marked with short and 

 stronger striae, placed at right angles to the longitudinal 

 striae : colour reddish-brown, mottled or streaked with 

 white ; margins as in P. opercularis ; slope below the 

 byssal sinus seldom, and never distinctly, serrate : beaks 

 prominent : ears nearly equal in the upper valve, but not 

 so in the lower valve, in which the right ear is the larger ; 

 all of them are sculptured by ribs radiating from the beak, 

 as well as by fine and numerous striae, which run from the 

 sides or shoulders and diverge outwards ; there are also 

 some wavy striae in the line of growth, crossing the auri- 

 cular ribs ; the ears of the lower valve project beyond and 

 overlap those of the upper valve, but much less than in 

 any of the preceding species ; byssal notch slight : hinge- 

 line straight : cartilage small : ligament very thin : hinge- 

 plate rather broad, microscopically striate across ; trans- 

 verse rib slight, and raised a little on each side of the car- 

 tilage-pit : inside fluted or grooved, as in other ribbed 



