Area.] PELECYPODA. 847 



KEY TO GENERA. 



A. Shell more or less subrhomboidal, hinge straight . . . . ARCA. 



B. Shell orbicular or suborbicular, hinge curved . . . . GLYCYMERIS. 



Genus 1. ARCA (Linne), Lamarck, 1799. 



Area, Linne, Syst, Nat,, ed. 10, 1758, (593 ; Lamarck. Prodrome, 1799, 87. 

 Type : A. noce, Linne. Daphne and Daphneoderma, Poli, 1795. Bysso- 

 arca, Swainson, 1835. Thyas, Gray, 1857. Cibota (Browne), Morch, 

 1852. Arcoptera, Heilprin. 



Animal having the foot long, bent, grooved, and with a byssus ; 

 gills long and narrow, oblique, deeper striated on the inner side ; 

 heart above the rectum. 



Shell commonly equivalve, subrhomboidal, ventricose, costulate 

 or cancellate ; borders of the valves either smooth or dentate ; hinge 

 straight, horizontal, furnished with very numerous short teeth ; 

 umbones prominent, curved inwards, separated from each other by 

 a more or less broad area, which carries several ligamental strise, or 

 well-marked narrow linear grooves commonly V-shaped ; impres- 

 sions of the adductor muscles of the valves subequal. the anterior 

 being rounded, the posterior divided ; impression of the posterior 

 adductor of the byssus elongate and situated near the cardinal line, 

 that of the anterior adductor of the byssus small ; pallial line simple. 



Distribution. About 150 species from all seas are known, the 

 maximum from the tropical seas ; a few are abysmal. 



The fossil species are numerous. 



Vernacular Name. Noah's Ark shell. 



KEY TO SUBGENERA. 



A. Shell equivalve ; teeth vertical in the middle , , . . BARBATIA. 



B. Shell inequivalve, left valve larger than the right ; teeth more or 



less oblique . . . . . . . . . . . . SCAPHARCA. 



Subgen. 1. BARBATIA (Gray), Adams, 1858. 



Barbatia, Gray, Synops. Brit, Mus., 1840, 151 ; ibid., 1844, 81. Type : 

 Area barbata, L. Barbatia, H. and A. Adams, Ad. G.R.M., ii, 185 53 



Shell oblong, longitudinal, or subquadrate ; surface of valves 

 covered with an epidermis, which is usually loose and rough. The 

 cardinal area is narrow, with numerous grooves for the resilium, which 

 form a series of elongated concentric lozenges on the area ; the shell 

 is not conspicuously truncate or keeled ; the teeth are small and 

 vertical in the middle of the series, and towards the end diverge distally 

 and become larger and more distant. Pallial impression entire. 



KEY TO SECTIONS. 



A. Ligament forming a series of elongated concentric lozenges on 



the area . . . . . . . . . . . . BARBATIA. 



B. Ligamental area narrow, leaving a free anterior and sometimes 



also posterior area . . . . . . . , . . ACAR. 



