190 / AMERICAN MARINE CONCHOLOGY. 
This species will probably prove to be a mere variety of the 
following. 
6. C. LATERALIS, Say. Fig. 523. 
(Mytilus.) Journ. Philad. Acad., ii. 264. 1822. 
Shell 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., 25. 1835. 
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. LEUcopHzATA, Conrad. Fig. 524. 
(Mytilus.) Journ. Philad. Acad., vi. 263, t. 11, f. 13. 1831. 
Shell incurved, with a very rugose, brownish epidermis; anterior 
side much depressed. Hinge.margins excavated, with the teeth 
obsolete. 
Chesapeake Bay, southwards. (Brackish water.) 
Family AVICULID&. 
Animal with the mantle-lobes free, their margins fringed; foot 
small, spinning a byssus; gills two on each side, crescent-shaped, 
entirely 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, Linn. 
