VARIETIES OF OYSTERS. 183 



shells highly coloured and streaked. Mr. Norman has 

 noticed, in his list of Clyde Mollusca, that the shells of 

 this species lose their azure blue colour after being kept in 

 a closed cabinet. This is remarkable, because certain 

 colours of shells and other animals (e.g., pink and reddish- 

 brown) fade, and even vanish, unless the light is excluded. 

 In the British Museum it has been found necessary to 

 replace, every two or three years, fresh specimens of many 

 delicate tinted butterflies exposed in the show-cases ; and 

 in the museum at Amsterdam the shells are always kept 

 covered, to prevent loss of colour. P. striatus attains a 

 greater size than P. Testa. My largest specimen is about 

 an inch long, and a trifle more than nine-tenths of an inch 

 in breadth. The striae in the present species are raised 

 and irregular, and they are never punctured or reticulate. 

 The different substance of the shell, the style and intensity 

 of coloration, and the comparative size of the left ear of 

 the upper valve are also notable marks of distinction- 

 Even the fry of each species exhibits its own peculiar 

 characters. 



It is the P. fuci of Gmelin, P. reticulatus of Chemnitz, 

 P. Landsburgi of Forbes, and P. aculeatus of Jeffreys. 



8. P. SIMILIS, Laskey. 



P. si?nilis, Lask. Mem. Wern. Soc. i, p. 387, t. 8, f, 8 ; F. 

 and H. 2, p. 293, pi. 52, f. 6, and (animal) pi. S. f. i. 



BODY very variable in colour, often pale yellow or 

 whitish, with brown streaks and blotches : mantle fawn- 

 coloured, with patches and lines of orange and black : 

 cirri short ; some are white and others brown, a few bein2f 



o 



thicker and longer than the rest, and mottled with milk- 

 white : ocelli 6-8 in front and nearly twice as many in a 



