MARINE MOLLUSCA OF THE UNITED STATES. 125 
teeth, or with very minute teeth; anterior muscular impression 
small and narrow, posterior large, obscure. _ 
AvicuLipa#. 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. 
OstraiID&. 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 PHOLADIDA. 
Synopsts of Genera. 
* Anterior hiatus always open. PHOLADINA. 
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. ZIRPHAA. 
** Anterior ventral gap closed in the adult by a callous plate. 
JOUANNETINA, 
With a single accessory valve. MaRrteEsiA. 
With two accessory valves, the principal plate over the umbones, with a 
smaller anterior one adjoining. DIPLOTHYRA. 
Genus PHOLAS, Linneus. 
_ 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, Linneus. Figs. 257-259. 
Syst. Nat., edit. xii. 1111. 1767. 
Shell very large, thin, inflated, with strong crenulate radiating 
ribs, about half an inch apart dn the basal margin, armed with 
vaulted scales caused by the elevation of growth strie. 
Vertical axis 2, transverse 7 inches. 
Whole Coast from New Bedford, Mass., to West Indies. 
