1014 PELECYPODA. [Eulamellibranchia. 



SUBORDER 7. ADESMACEA. 



Eulamellibranchia with very long united siphons and a largely 

 closed mantle. The foot short, truncated, discoid, and without a 

 byssus. The gills prolonged into the branchial siphon. The shell 

 gaping and devoid of a ligament, but with a styloid apophysis in the 

 umbonal cavities. 



Fam. PHOLADIDJE, Adams. 



Animal with the edges of the mantle closed, except for a pedal 

 aperture ; siphons large, elongated, united nearly to their ends ; 

 orifices fringed ; gills narrow, prolonged into the branchial siphon, 

 attached throughout, closing the branchial chamber ; the heart 

 traversed by the rectum : 2 aortse ; foot short and truncated. 



Shell cellular-crystalline, with a thin epidermis ; valves more or 

 less gaping in front and behind, with inconspicuous beaks and reticu- 

 late often spinose sculpture ; in the adult supplemented by accessory 

 shelly pieces, always attached to the valves, but not by an exterior 

 shelly tube ; the antero-dorsal margins more or less extensively 

 reflected, the postero-ventral approximated ; pallial line sinuated ; 

 area none ; ligament and resilium usually absent, an obsolete remnant 

 of the resilium and chondrophore sometimes present in the left valve. 



The animals of this family are living in calcareous rocks, clay, or 

 submerged wood. During growth the enormous foot is chiefly em- 

 ployed in grinding away the sides of the burrow, which is usually 

 vertical and symmetrical. The animals are vividly phosphorescent 

 in the dark. 



Jura to Recent. 



KEY TO GENERA. 



A. Shell with the anterior gape closed in the adult ; valves with a 



median groove extending obliquely from the beaks to the 

 basal margins, and a posterior horny or calcareous cusp 

 (siphonoplax) . . . . . . . . . . PHOLADIDEA. 



B. Shell with the anterior gape always open ; valves without 



median groove ; no siphonoplax . . . . . . BARNEA. 



Genus 1. PHOLADIDEA, Goodall, 1819. 



Pholadidea, Goodall, in Turton, Conch. Diet., 1819, 147. Type : P. Los- 

 combiana, Goodall. Cadmusia, Leach, 1852. 



Animal with the siphons united to their ends, where they are 

 surrounded by a fringed disc ; a simple anal valve ; palpi long and 

 narrow ; gills very unequal ; foot large and truncated in the young, 

 atrophied and rudimentary in the adult. 



Shell equivalve, transversely elongated, convex, gaping in front in 

 the young, but closed by a callous plate in the adult ; outer face of 

 the valves divided by a groove extending from the umbones to the 

 ventral margins ; with a double anterior accessory plate (protoplax), 

 the other plates (mesoplax and metaplax) present or absent ; the 



