VARIETIES OF OYSTERS. l6l 



SHELL more or less inequivalve : ligament internal : 

 muscular scar nearly central. 



The name of this genus is nearly as ancient as that of 

 Ostrea, It is very expressive, the shell usually having ribs 

 which are arranged like the teeth of a lady's comb. Some- 

 times it resembles the expanded frame of a fan. Scallops 

 are especial favourites of shell-collectors and amateurs, on 

 account of their elegant shape and their brilliant and 

 varied hues. The curious organs called "ocelli" or eye- 

 lets are supposed by some physiologists to be rather highly 

 organized, and even superior to the so-called eyes of most 

 Gasteropodous Mollusca. More than one hundred of them 

 have been counted in a single individual of some species 

 of Pecten. For this reason Poli called the animal Argus. 

 These little eyes have a prismatic lustre, and gleam like 

 precious stones which are set round the inside of a casket 

 lined with mother-of-pearl. Their structure has been 

 lately and independently investigated by Grube, Khron, 

 and Will. Very young shells of all the species are desti- 

 tute of ribs, and they are nearly rhomboidal, owing to their 

 breadth and the size of their ears being proportionally 

 greater at that stage of growth than afterwards is the case. 

 In consequence of the scallops being generally attached or 

 sedentary, the upper valve is more deeply and brightly 

 coloured than the lower one. 



Although all the essential characters of the present 

 genus are uniform and do not vary much in the several 

 species, it has been divided by authors into no less than 

 twenty-eight, most of which will be found enumerated in 

 the useful Index of Herrmannsen. In nearly all the British 

 species the upper or left valve is the larger, and is also dis- 

 tinguished from the other by its brighter or deeper hue. 



