962 



PELECYPODA. 



[Eula mfllibm nckin . 



at an angle above, and usually more or less solidly united at this line 

 of junction ; behind them the resilifer, and above the scar of the 

 ligament. 



KEY TO GENEEA. 



A. Dentition normal, as described for the subfamily ; laterals smooth 



or gramilar ; ligament separated from resilium by a shelly 



lamina.. .. .. .. .. .. .. MACTRA. 



B. Left cardinal with a small posterior accessory lamella, both pro- 



jecting over the resilifer ; laterals transversely grooved ; liga- 

 ment not separated from the resilium by a shelly lamina . . SPISULA. 



Genus 1. MACTRA, Linne, 1767. 



Mactra, Linne, Syst. Nat., ed. 12, 1767, 1125; Lamarck, 1799. Type: 

 M. stultorum, L. Trigonella, Da Costa, 1778 ; not of Walch, 1702. or 

 Schroter, 1776. Crassatella, Lamarck, 1799. 



Animal with thick papillate mantle -borders, mantle open in front ; 

 siphons united to their extremity, partly covered by a chitinous sheath, 

 the orifices fringed, the anal opening with a tubular valve ; foot 

 elongated, large, linguiform, not byssiferous ; labial palps long, tri- 

 angular ; branchiae unequal, the outer narrower and appendiculate. 

 Shell ovate - trigonal ; surface smooth or concentrically striated. 

 Dentition normal in number and distribution of teeth ; ligament 

 set off by a shelly lamina rising between chondrophore and liga- 

 ment ; cardinals generally coalescent above ; laterals smooth or finely 

 granular. Pallial sinus round or angulate. 



Distribution. About 150 species from all seas. 



Fossil in the Cretaceous and Tertiary, the maximum in the Mio- 

 cene. 



Vernacular Name. Trough-shell. 



SYNOPSIS OF SUBGENEEA. 



A. Shell equilaterally oval ; pallial sinus high, rounded in front ; 



ligament completely shut off from the resilium ; resilifer 

 roofed apically ; laterals long ; anterior sinus with a but- 

 tress upon which stands the anterior arm of the cardinal ; 

 anterior right cardinal adjacent to the dorsal shell- margin 



B. Shell inequilateral, with coarse epidermis ; ligament lanceo- 



late, sunken ; teeth and laminse short ; anterior right 

 cardinal lying in the plane of the ventral lamina 



C. Shell subequilateral, elongate ; pallial sinus large ; valves 



gaping ; resilifer large, shallow, apically roofed ; cardinals 

 prominent, their posterior arms projecting over the 

 resilifer, each anterior arm with an accessory lamella in 

 nearly the same plane, appressed to the lateral 



CCELOMACTEA. 



MACTEODEBMA. 



MACTROTOMA. 



Subgen. 1. CCELOMACTRA, Call, 1895. 



Ccplomacira, Dall, P. Mai. S., i, 1895, 211. Type: Mactra violacea (Ch.), 

 Gmelin. Ccelomactra, Dall, Trans. Wagn. Free Inst., iii, pt. 4, 1898, 875. 



Shell equilaterally oval, thin, inflated, with a thin lineated epi- 

 dermis ; dorsal areas grooved ; beaks adjacent ; pallial sinus very 

 short, high, rounded ; valves nearly close-fitting, convex ; ligament 



