70 BIVALVES_— MYTILUS. 
inspection of the hinge, the error will soon be dis- 
covered. 
Muscles differ greatly in external appearance, some 
being perfectly smooth, and beautifully marbled and 
variegated with delicate colors; others elegantly ra- 
diated with purple and white; and a third class 
bemg of one color only, as black, blue, green, yellow, 
or brown, and coarsely ribbed and grained with mi- 
nute tubercles. In the greater part the coloring is 
confined to the epidermis; when this is removed, and 
the shell polished, so different a surface is presented, 
that an adept would be puzzled to decide to what species 
it should be referred. In some specimens the epidermis 
is shaggy or bearded. 
Many exhibit internally a pearly appearance, and 
some, when uncoated and polished, display considerable 
brilliancy. The M.margaritiferus is celebrated for its 
iridescent colors, and is, moreover, valued for the beau- 
tiful and costly pearls it produces. The young shells 
of this species are sometimes so different to the adults 
in appearance, that they can scarcely be recognised as 
bemg the same. 
The M. edulis is found in immense beds or layers, and 
invariably affixes itself to other bodies by means of its 
silky byssus. 
The M. cygneus and M. anatinus, (both fresh-water 
species), frequently become the prey of birds: when 
the shell is too hard to be penetrated by their beaks, 
they mount with it to a considerable height, and by let- 
ting it fall, are enabled to pick out the fish from the 
broken shell. 
