190 AMERICAN MARINE CONCHOLOGY. 



This species will probably prove to be a mere variety of tlie 

 following. 



6. C. LATERALIS, Say. Fig. 523. 



iMi/iilus.) Jouru. Philad. Acad., ii. 264. 1823. 



Sliell transversely suboval, inflated, subpellucid, with numerous 

 concentric wrinkles ; ribs alternately larger and smaller ; shell 

 inflated from the beak to the posterior basal angle ; epidermis 

 greenish or brownish. 



Southern Coast. 



Genus DREISSENA, Van Beneden. 

 Bull. Brux. Acad., 2o. 183.>. 



Animal with closed mantle ; byssal orifice small, and siphon 

 very small, conical, plain, branchial prominent, fringed inside ; 

 palpi small, triangular. 



Inhabits brackish or fresh waters. 



1. D. LEUCOPH^TA, Conrad. Fig. 524. 



{Mytilus.) Journ. Philad. Acad., vi. 263, t. 11, f. 13. 1831. 



Shell incurved, with a very rugose, brownish epidermis ; anterior 

 side nciuch depressed. Hinge.margins excavated, with the teeth 

 obsolete. 



Chesapeake Bay., southwards. (Brackish water.) 



Family AVICULID.E. 



Animal with the mantle-lobes free, their margins fringed ; foot 

 small, spinning a b3'ssus ; gills two on each side, crescent-shaped, 

 entirel}' free, or united to each other posteriorly or to the mantle. 



These shells are natives of tropical and warm seas ; no living 

 species are found in northern latitudes. 



Synopsis of Genera. 



Shell obliquely oval, very inequivalve ; right valve with a byssal sinus 

 beneath the anterior ear ; cartilage pit single, oblique ; hinge with one or 

 two small cardinal teeth, and an elongated posterior tooth, often obsolete ; 

 posterior muscular impression (adductor and pedal) large, sub-central ; 

 anterior (pedal) scar small, umbonal. Avicula, Brug. 



Shell equivalve, wedge-shaped ; umbones quite anterior ; posterior side 

 truncated and gaping ; ligamental groove linear, elongated ; hinge eden- 

 tulous ; anterior adductor scar apical, posterior sub-central, large, ill- 

 defined ; pedal scar in front of posterior adductor. Pinna, Linu. 



