MARINE MOLLUSCA OF THE UNITED STATES. 185 
7. L. cAscornsis, Mighels. Figs. 505-7. 
(Nucula.) Bost. Journ. Nat. Hist., 40, t. 4, f. 6. 
Shell ovate, rather thin, finely striate, slightly inequilateral ; 
anterior side semi-oval; posterior side tapering nearly to a point, 
with a well-defined areola, sharply compressed, with a slight wave 
below the areola; epidermis greenish straw-color; beaks small, 
nearly central; teeth small, 10 anterior and 10 or 12 posterior. 
Length 15, height 9 mill. 
Casco Bay, Maine. 
Family MY TILID. 
Animal marine (or sometimes fluviatile), attached by a byssus; 
mantle-lobes united between the siphonal openings; gills two on 
each side, elongated, and united behind to each other and to the 
mantle; dorsal margins of the outer and innermost laminee free ; 
foot cylindrical, grooved. 
Synopsis of Genera. 
Shell wedge-shaped, rounded behind, wmbones terminal, pointed ; hinge- 
teeth minute or obsolete ; pedal muscular impressions 200 in each valve, 
small, simple, close to the adductors. Mytiuvs, Linn. 
Shell oblong, inflated in front; wmbones anterior, obtuse ; hinge toothless ; 
pedal impressions three in each valve, the central elongated ; epidermis 
sometimes produced into long beard-like fringes. Moproua, Lam. 
Shell cylindrical, interior nacreous; otherwise like Modiola. 
LirHopomvts, Cuy. 
Shell short, ovate, partly smooth, and partly ornamented with radiating 
strie ; hinge margin crenulated behind the ligament ; interior brilliantly 
nacreous. CRENELLA, Brown. 
Shell ovate, oblong, obtusely keeled right valve with a slight byssal sinus ; 
beaks terminal, furnished internally with a transverse shelf or septum ; 
hinge composed of an imperfectly developed cardinal tooth in the right 
valve, with a corresponding socket in the left; ligament linear, internal ; 
pedal impression single, posterior. Fluviatile. 
DREISsENA, Van Beneden, 
Genus MYTILUS, Linnzus. 
Syst. Nat., edit. x., 705. 1758. 
The common edible mussel frequents mud-banks which are un- 
covered at low water; the fry abound in water a few fathoms 
deep; they are full-grown ina single year. Pieces of wreck are 
frequently covered by mussels of all ages, attached by their byssus. 
There are about sixty-five species, of world-wide distribution. 
