OYSTERS AND SCALLOPS. 225 
ORDER I. MONOMYARIA. 
FAMILY OSTREADZ. 
(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 de- 
velopment of organization, and, above all, the esti- 
mation 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 con- 
cealed 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 (Zima) and the Scallops (Pecten) are 
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