MYTILUS. og 
margins are connected by short close-set perpendicular mus- 
cular threads, which act not only by the lines of junction, but 
also by a general subcircular contraction and dilatation of the 
fibres of connexion into a still closer or more lax union ; the 
effect of this combined action is to throw out and withdraw 
the tentacular fringes of the posterior lateral and ventral 
portions of the mantle; these vertical muscular lines also 
supply the adherence of the mantle to the margins of the 
shell, as in this species there are no longitudinal muscular 
bands emanating from the adductor muscles to support it. 
The foot is thick, fleshy, narrow, moderately long, proceed- 
ing straight from the heel, of an almost black-brown, marked 
with occasional lateral white lines ; it produces coarse light 
brown byssal filaments from a groove at the posterior under 
part which is continued to the point; the upper surface is 
pla, not grooved. The animal can detach itself from the 
byssus, by extracting and leaving it fixed to the substance on 
which it is moored, and in a very short time it is replaced by 
another ; this is the operation with the free Mytili. It is 
almost needless to remark that the great mass of these ani- 
mals are closely impacted in the sublittoral and laminarian 
districts. 
There are a pair of light brown branchiw, of similar form, 
depth, and position, which run the length of the dorsal range, 
passing their diminished and well-fixed posterior ends into the 
branchial aperture; they are scarcely visibly pectinated on 
either side; they also anteally diminish in depth, curving to 
the pair of palpi on each side the mouth ; they are connected 
by narrow labia, and at the basis with the palpi, which are 
long, very thick, triangular, pointed, of the same colour as the 
branchiz, each having in the centre a raised line, probably an 
artery or branchial vein, from which they bevel on each side 
to a sharp edge, each pair folded together, being well striated 
within, not “ partially,” as is stated in the ‘ British Mollusca’ 
on my authority, and plain on the outer surface. The liver is 
pale yellowish green, and granular. The crystalline stylet and 
stomachal attritor are present. The lateral dorsal secreting 
glands, which I believe produce the cartilage and a portion 
