MARINE MOLLUSCA OF THE UNITED STATES. 125 



teeth, or -with very minute teeth ; anterior muscular impression 

 small and narrow, posterior large, obscure. 



AvicuLiD^. Shell inequivalve, very obliquely resting on the 

 smaller (right) valve, and attached by a byssus ; posterior muscu- 

 lar impression large, subcentral, anterior small, within the umbo ; 

 pallial line irregularly dotted ; hinge line straight, elongated ; 

 umbones anterior, eared, the posterior ear wing-like ; cartilage 

 contained in one or several grooves ; hinge edentulous or obscurely 

 toothed, 



OsTRiEiD^. Shell inequivalve, slightly inequilateral, free or ad- 

 herent, resting on one valve ; beaks central, straight ; ligament 

 internal ; epidermis thin ; adductor impression single, behind the 

 centre ; pallial line obscure ; hinge with or without primary teeth. 



Family PHOLADID^. 



Synopsis of Oenera. 



* Anterior hiatus always open. Pholadin^. 



Dorsal valves placed anterior and posterior to the beaks, umbonal processes 

 reflected over the beaks, closely applied. Pholas. 



Dorsal valves lanceolate, placed side by side. Umbonal processes reflected 

 over the beaks, cellular beneath. Dactylina. 



Destitute of accessory valves. . Zibph^a. 



** Anterior ventral gap closed in the adult by a callous plate. 



Jouannetin^. 



"With a single accessory valve. Martesia. 



With two accessory valves, the principal plate over the umbones, with a 

 smaller anterior one adjoining. Diplothtra. 



Genus PHOLAS, Linnaeus. 

 Syst. Nat., edit. x. 1758. 



There are but four recent species of Pholas known, as now re- 

 stricted, and they are very easily distinguishable from each other. 



* Margins of the valves regularly rounded anteriorly. P. costata. 

 ** Anterior ventral margin emarginate. P. truncata. 



1. P. COSTATA, Linnaeus. Figs. 2.57-259. 

 Syst. Nat., edit. xii. 1111. 1767. 



Shell very large, thin, inflated, with strong crenulate radiating 

 ribs, about half an inch apart on the basal margin, armed with 

 vaulted scales caused by the elevation of growth strise. 



Vertical axis 2, transverse 7 inches. 



Whole Coast from New Bedford, Mass., to West Indies. 



