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 4, 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. 
