952, ARCID A. 
Shell bivalve, equivalve, inequilateral, transversely elongated, 
and more or less recurved with depressed convex valves, which 
have an antero-mesial constriction, slightly angular umbonal, 
and somewhat prominent incurved subanterior beaks; cardinal 
line recurved, bordered by a narrow escutcheon and lunette ; 
posterior end truncate and usually nearly rectangular to the 
posterior part of the cardinal line. Surface marked by moder- 
ately strong irregular concentric undulations on the anterior 
end, which extend to the umbonal ridge and sometimes continue 
vertically across the cardinal slope. Some species are likewise 
marked by fine radiating lines of pustules on the body of the 
shell. Cardinal slope often marked by one or more radiating 
ridges; valves united by an external ligament. Hinge-teeth and 
muscular impressions undetermined. 
? DysracreLLA, Hall and Whitfield, 1872 
Type, Tellinomya subnasuta, H.and W. Fossil; U. Helder- 
bere, Kentucky. 
Shell unequally ovate, twice as long as high, with very ventri- 
cose valves, giving a subeylindrical form anterior to the beaks ; 
posterior end very narrow, pointed at the extremity ; anterior 
end broadly rounded, longest above the centre, basal line very 
slightly insinuated, beaks small, appressed, at two-thirds the 
entire length from the anterior extremity ; muscular impressions 
moderate in size, distinctly marked, situated near the margins ; 
pallial line entire, composed of a series of radiating pustules, as 
seen on the cast; crenulations of the hinge not distinctly seen, 
but the evidence possessed would indicate them to have been 
minute. 
Famity ARCIDA. 
Shell regular, equivalve, with strong epidermis; ligament 
exterior, occupying an area between the beaks; hinge with a 
long row of similar, comb-like teeth; pallial line ‘distinct ; mus- 
cular impressions subequal. Structure corrugated, with vertical 
tubuli in rays between the ribs or strize —CARPENTER. 
Animal with the mantle open; foot large, bent, and deeply 
grooved ; gills very oblique, united posteriorly to a membranous 
septum. 
Arca, Linn. 
Etym.— Arca, a chest. 
Syn.—Navicula, Blainy., 1818. Byssoarea, Swains., 1840. 
Daphnzeoderma, Poli, 1792. 
Distr.—140 sp. World-wide, most abundant in warm seas; 
low water—230 fathoms (A. ¢mbricata, Poli). Prince-Regent 
Inlet (A. glactalis). Fossil, 400 sp. Lower Silurian—; United 
States, Europe, South India. A. Noe, Linn. (exxvi, 39). 
