MYTILUS. 50 



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 

 plain, 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 My till. 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 branchiae, 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 

 branchiae, 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 



