MARINE MOLLUSCA OF THE UNITED STATES. 185 



7. L. CASCOENSIS, 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 MYTILID^E. 



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 laminae free ; 

 foot cylindrical, grooved. 



Synopsis of Genera. 



Shell wedge-shaped, rounded behind, umbones terminal, pointed ; hinge- 

 teeth minute or obsolete ; pedal muscular impressions two in each valve, 

 small, simple, close to the adductors. MTTILUS, Linn. 



Shell oblong, inflated in front ; umbones anterior, obtuse ; hinge toothless ; 

 pedal impressions three in each valve, the central elongated ; epidermis 

 sometimes produced into long beard-like fringes. MODIOLA, Lam. 



Shell cylindrical, interior nacreous ; otherwise like Modiola. 



LlTHODOMUS,* CUV. 



Shell short, ovate, partly smooth, and partly ornamented with radiating 

 striae ; 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 MYTILTTS, Linnaeus. 

 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 in a 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. 



