MUSSELS. 255 
fined localities it 1s often much distorted, but free 
specimens are very regular.” * The epidermis in 
this species often projects beyond the edges of the 
valves, in the form of a ragged fibrous fringe. 
FAMILY MytTILID&. 
(Mussels.) 
The members of this family are almost all 
marine, though one genus (Dretssena) inhabits the 
rivers of Eastern Europe, and is now found plenti- 
fully in some of the navigable streams of this 
country, having been introduced, as is supposed, 
adhering to timber, or to the bottoms of ships. 
The distinctive characters of the family, as enume- 
rated by M. de Blainville, are as follows :— 
The shell is regular, with the valves equal but 
much lengthened, and produced on one side. The 
hinge is destitute of teeth ; the ligament dorsal and 
linear. ‘The texture of the shell is sometimes thin 
and horny, and it is often covered with an epider- 
mis. ‘There are two muscular impressions, the 
hinder of which is considerably larger than the 
other. 
In the animal we find the mantle adhering to- 
wards the borders, but open throughout its inferior 
portion, with a distinct anal tube, but the branchial 
one rudimentary, and indicated only by a thicken- 
ing of the posterior borders of the mantle. The 
foot is narrow and tongue-shaped, grooved down 
its centre, and furnished with a byssus backwards 
at its base. In some of the genera, as Lithodomus, 
a foreign group which burrow in the hardest rocks, 
* Forbes and Hanley. 
