BIVALVIA. 7 



upper, while one only is formed upon the lower valve, the other two passing into the 

 calcareous operculum by which it is fixed. Sexes distinct. 



As the individuals of this genus are always attached, they are seldom of a regular 

 form, but generally more or less distorted, modified by, and often assuming the shape 

 and characters of the body to which they adhere ; and as they are frequently attached 

 to the shells of the Pecten, an individual of this genus, which in its natural state is 

 nearly smooth, will become, in consequence, rayed or pectinated, partaking of the 

 characters of the body it has been living upon. If, therefore, it be attached near the 

 umbo of the Pecten, its regular increase will assume the form of that genus ; but if its 

 attachment be upon the wider rays, these ribs will not represent the regular form, but 

 the impress of its place of attachment will be shown, as in fig. 3 b, in parallel or nearly 

 parallel ridges across the shell. In order to produce this appearance, the addition 

 that is made by the mantle to the edges of the shell are carried over the ribs of the 

 Pecten down into the interspaces, by which means a costated form is given to a shell, 

 otherwise smooth. This character, however, according to Mr. Clark, appears to be 

 eclectic, or at the will of the animal. Thus, whenever the under or lower valve has 

 its edges elevated above the ribs of the Pecten, so as not to be influenced by those 

 inequalities, then the upper valve retains its original form. The lower valve is 

 generally thin, often papyraceous, so that in the fossil state, the upper valve is 

 the most numerous. 



The Anomia is closely allied to the Pectens, and the perforation in the lower valve* 

 is said by the authors of the ' Hist, of Brit. Mollusca,' to be chiefly a greater extension 

 of the auricular sinus of that genus ; and that the young fry will be probably found 

 attached by means of a byssus, which as the animal increases, eventually becomes 

 converted or transformed into the calcareous opercular process of the older shell, this 

 organ of attachment being merely the extension and indurated portion of the lower 

 part of the adductor. 



A large number of detached valves are found in the Coralline Crag, but their 

 specific appropriation is a matter of great difficulty from their excessive variability of 

 form, as well as great irregularity in their external ornament ; and as their correct 

 assignment, even in a recent state, with " all appliances and means to boot" by the 

 aid of their animal inhabitant, as well as by assistance given in the colouring matter of 

 the shell, is still a doubtful matter, the appropriation of the fossil species may be looked 

 upon with suspicion. 



Mr. Clark in the examination of this genus, has arrived at the conclusion, that 

 there is but one species now found in the British Seas ; and that the extraordinary 

 variation both in form and sculpture, exhibited by individuals, is so fluctuating in 

 character, as not to be depended upon for specific distinction. As, however, there 

 are generally some marked differences in these shells by which the variations may be 

 separated, I have followed the authors of ' British Mollusca,' in considering them for 

 the present so many distinct species. 



