252 DIMYARIA.—ARCADA,. 
ORDER II. DIMYARIA. 
FamIty ARCADA., 
(Arks.) 
A WELL marked character of this family, and one 
that a glance is sufficient to recognise, is the pecu- 
liarity of the hinge. This, whether straight or 
bent, extends along a large portion of the margin 
of each valve, and is formed of a great number of 
minute comb-like teeth arranged in a row, nearly 
parallel to each other, or slightly diverging. Those 
of one valve correspond in shape and size to those 
of the other, with which they accurately interlock. 
The beaks (wmbones) are generally distant from 
each other when the shell is closed. In form, size, 
solidity, sculpture, and colour, there is much diver- 
sity in the species; but all are covered with an 
epidermis. ‘Two distinct muscular impressions 
are strongly marked, which are remote from each 
other, but are connected by a line running parallel 
to the border of the valve, which is the pallial im- 
pression. 
The animal has the mantle freely open, without 
siphonal tubes, except in a few species. It is 
partially adherent to the shell; sometimes pro- 
longed backwards. The foot is always considerably 
developed, deeply grooved, and capable of expand- 
ing into a disk, like that of a Gasteropod. 
All the species are marine, and inhabit various 
