234 MALACOZOA. TROP1OPODA. LAMELLIBKANCHIATA. 



GENUS 1. MYTILUS. MUSSEL. 



Animal oblong. The lobes of the mantle adherent 

 and thickened at the margin, with an internal crenate or 

 fringed edge, united posteriorly only so as to form an 

 anal siphon. Foot slender, linguiform, with a median 

 groove beneath, a pair of cylindrical retractor muscles, 

 two anterior and four posterior muscles, and furnished 

 at its base behind with a silky byssus. On each side a 

 pair of very thin, nearly equal branchiae. Mouth rather 

 large, with two pairs of thin lanceolate palpi. Anterior 

 adductor muscle very small, posterior larger and cylin- 

 drical. 



Shell elongated oblong or obovate, equivalve, extremely 

 inequilateral, convex, concentrically striated, often longi- 

 tudinally grooved. Umbones small, prominulous, some- 

 what incurved. Hinge toothless, often with small 

 marginal lamellse. Ligament internal, dorsal, very long, 

 linear. Dorsal margin nearly straight, frontal obsolete, 

 the umbones being terminal, inferior margin much pro- 

 longed and nearly straight, posterior rounded. 



Attached to rocks or other bodies by the byssus. ; 



1. Mytilus edulis. Eatable or Common Mussel. 



Shell obovato-oblong, with the dorsal line forming a rather 

 prominent angle behind, the anterior side obsolete, the anterior 

 part of the ventral line nearly straight, the posterior ventral 

 extremity rounded; the epidermis dusky, glossy, generally 

 somewhat iridescent ; the surface of the shell blue, or rayed 

 with blue and white, sometimes as well as the epidermis, reddish, 

 or yellowish, the inside bluish-white, with the margin dark 

 blue ; three or four small lamellae or crenatures on the anterior 

 margin close to the hinge. Length four inches, height two 

 inches and two-twelfths. 



It varies greatly in form, some individuals being very long 

 and narrow, others broad ; some straight, others considerably 

 curved. Young shells often yellowish or reddish and pellucid. 

 In some localities several varieties are intermixed. On ex- 

 posed parts of the coast they are often stunted, shortened, and 

 incurvate, with the epidermis abraded, and the surface rough. 



