PECTEN. 51 



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

 1. Pecten pu'sio"*, Linne. 



Ostrea pusio, Linn. Syst. Nat. p. 1146. T.^usio, F. & H. ii. p. 278, pi. L. 

 f. 4, 5, and li. 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 con- 

 sequence of its fitting the cavities and sinuosities 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 transverse 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 

 stria3 only are visible, the ribs not then existing : colour red- 

 dish, 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 

 outwards ; 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 



* A youngster. 



D 2 



