PHOLABACEA. 323 



Order PHOLADACEA. 



Mantle closed, provided with two contiguous, more or 

 less elongated siphonal tubes, which are usually united; 

 gills two pairs, produced into the lower or branchial 

 siphon; pedal opening small. Foot frequently more or 

 less elongated and club-shaped. 



Fam. PHOLADID^. 



Animal symmetrical, club-shaped or worm-like. Palpi 

 elongate, linear. Mantle partly exposed, closed in front, 

 except an aperture for the foot ; siphons large, elongated, 

 united nearly to their ends, orifices fringed ; gills narrow, 

 prolonged into the branchial siphon, attached throughout, 

 closing the branchial chamber. Foot short and truncated. 



Shell free, or within a tube ; valves equal, gaping at both 

 ends, thin, white, brittle, armed in front with rasp-like 

 imbrications, without hinge-teeth, and strengthened ex- 

 ternally by accessory valves ; hinge-plate reflexed over the 

 beaks, and furnished with a long, curved, muscular process 

 beneath each ; anterior muscular impression on the hinge- 

 plate ; pallial sinus very deep. 



Living perpendicularly in holes in rock or sand. 



The cartilage of the hinge in these shells is small and 

 internal ; the ligament is strong and elastic, situated ex- 

 ternally, and both are further strengthened by an accessory 

 membrane formed by the coriaceous end of the mantle, 

 which issues between the anterior ends of the valves and 

 covers the ligament ; this extension of the mantle is fixed 

 by filaments which enter the dorsal cells, and is furnished 



