252 ARCID-iE. 



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 bj^ 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 

 verticals 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 moi-e radiating 

 ridges ; valves united by an external ligament. Hinge-teeth and 

 muscular impressions undetermined. 



? Dystactella, Hall and Whitfield, 1812. 



Tj'pe, Tellinomya subnasula, H. and W. Fossil ; U. Helder- 

 berg, Kentucky. 



Shell unequall}^ ovate, twice as long as high, with very ventri- 

 cose valves, giving a subcylindrical 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. 



Family ARCID^. 



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 stride — Carpenter. 



■Animal with the mantle open ; foot large, bent, and deeply 

 grooved ; gills very oblique, united posteriorly to a membranous 

 septum. 



Arc A, Linn. 



Etym. — Arca^ a chest. 



Syn. — Navicula, Blainv., 1818. Bj^ssoarca, Swains., 1840. 

 DaphntBoderma, Poll, 1792. 



Disir. — 140 sp. World-wide, most abundant in warm seas ; 

 low water — 230 fathoms {A. imbricata, Poll). Prince-Regent 

 Inlet (A. glacialis). Fossil, 400 sp. Lower Silurian — ; United 

 States, Europe, South India. A. Noae, Linn, (cxxvi, 39). 



