ORDER II. DIMYARIA.
Family Arcadæ.
(Arks.)
A well marked character of this family, and one that a glance is sufficient to recognise, is the peculiarity 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 (umbones) are generally distant from each other when the shell is closed. In form, size, solidity, sculpture, and colour, there is much diversity 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 impression.
The animal has the mantle freely open, without siphonal tubes, except in a few species. It is partially adherent to the shell; sometimes prolonged backwards. The foot is always considerably developed, deeply grooved, and capable of expanding into a disk, like that of a Gasteropod.
All the species are marine, and inhabit various
