256 DIMYARIA:—MYTILIDA. 
the faculty of spinning is confined to the early life 
of the animal, but in others a strong and copious 
byssus is formed at pleasure through life. 
GENUS MYTILUS. 
The name of this familiar shell-fish is derived 
from the obvious resemblance which its shell bears 
to a mouse, the united beaks representing the nose. 
The Greeks used the term Més for both the shell 
and the quadruped, and both the Latin term Myti- 
lus and our own Mussel are derived from that 
appellation. 
The genus is characterised by a shell very un- 
equal-sided, but equal-valved, somewhat triangular, 
running off into pointed terminal beaks, the valves 
swelling, the surface covered with a horny skin ; 
the hinge, though sometimes notched, has no true 
teeth ; the narrow ligament is internal. 
The animal is lengthened and oval, the lobes of 
the mantle are simple or fringed, united behind in 
a single point, so as to form an anal siphon. The 
mouth is rather large, furnished with two pairs of 
soft lips. The foot is slender, strap-shaped, grooved, 
carrying at its base a silky byssus of considerable 
strength. 
The Edible Mussel (Mytilus edulis) is the only 
British species, but is too abundant and too well 
known to need description. Sold in every town 
as an article of food, its three-sided shells, black 
without and blue within, and the bearded animal, 
both in its raw and cooked state, are familiar to 
every one. 
Dr. Knapp of Edinburgh has communicated to 
Messrs. Forbes and Hanley a very interesting 
