I 62 OYSTERS, AND ALL ABOUT THEM. 



In Pecten maocimus, however, and occasionally in P, septem- 

 radiatus, the lower or right valve is the larger, and almost 

 or quite colourless. The intensity of colour is supposed 

 to depend on the action of solar light, although it is not 

 wanting in animals living in the abysses of the ocean, 

 which the most attenuated sunbeam has never directly 

 penetrated. 



A. Upper valve more or less convex : hinge-line 

 ribbed across. 



i. PECTEN PUSIO, Linne. 



Ostrea pusio, Linn. Syst. Nat., p. 1146. P. pusio, F. and 

 H. 2, p. 278, pi. L. f. 4, 5, and 51, f. 7. 



BODY vermilion or yellowish-white, with a brown 

 tint, or particoloured : cirri numerous, short, and blunt, 

 arranged in from 5 to 7 rows : ocelli large and few in 

 number. 



SHELL varying in shape according to age, being when 

 young considerably longer than broad, and regular, but in 

 its adult state broader in proportion, and distorted or 

 twisted in consequence of its fitting the cavities and sinu- 

 osities of the bodies to which it is fixed ; in the earlier 

 stage of growth it is almost equivalve, but afterwards the 

 upper valve becomes usually the larger and more convex 

 of the two ; sides nearly equal ; it is rather solid, and not 

 glossy: sculpture, about 70 narrow and sharp ribs, which 

 are alternately large and small, crossed by numerous trans- 

 verse plates, which by their intersection form scales or 

 prickles on the crests of the ribs ; the whole surface is 

 exquisitely marked by microscopical longitudinal striae, 

 which diverge from each successive layer of growth ; in 

 the fry these striae only are visible, the ribs not then 



