ORDER I. MONOMYARIA.
Family Ostreadæ.
(Oysters and Scallops.)
On account of the beauty possessed by many members of this family, (a beauty comprising elegance of form, elaborate sculpture, and the most rich and brilliant colours,) their high development of organization, and, above all, the estimation in which they are held, and the extent to which they are consumed as human food, this must be considered as the most important of all the families of the Conchifera, or even of the Mollusca. Considerable variation, indeed, is found in the anatomy of these animals, and some of them entirely lack those organs which are found well developed in others; yet they possess so many characters in common, and glide into each other by gradations so close, that it is difficult to divide the group. All the tribe have the mantle widely open, no siphonal tubes, a single adductor muscle, and a ligament either entirely or in part concealed in the edge of the shell, lodged in a cardinal groove, and sometimes accompanied with teeth. In most cases there is a minute foot capable of spinning those threads which are called byssus, but this power is for the most part exercised only while the animal is young.
The Limes (Lima) and the Scallops (Pecten) are
