334 gastrocilenidjE. 



Fam. GASTKOCH^ENID^. 



Animal symmetrical, elongated or claviform. Siphons 

 very long, contractile, united almost to their extremities, 

 orifices fringed with cirrhi ; mantle-margins united and 

 thickened in front, pedal opening small, anterior; gills two 

 on each side, narrow, prolonged into the branchial siphon. 

 Foot small, cylindrical, anterior, not byssiferous. 



Shell equivalve, gaping ; valves thin. Hinge-teeth rudi- 

 mentary; cartilage external, marginal, small, weak. Adductor 

 scars two ; pallial line sinuated. 



Living enclosed in calcareous tubes, or imbedded in mud 

 or stone, the shell-cases themselves being either free or 

 imbedded. 



The Gastrochanida together with the Pholades, constitute 

 the Tubicola of Lamarck, a very natural group characterised 

 by their living chiefly enclosed in tubes or burrows, which they 

 never leave. The valves of the shell are sometimes firmly 

 incorporated with the protecting tube, as in Brechites or 

 Aspergillum, or one valve only is free, as in Clavagella, or 

 else both valves are free, as in GastrocJuena. The tubes 

 themselves are usually buried, with the thickest end down- 

 wards, in the mud or sand near low-water mark. 



Genus GASTROCHJENA, Spengler. 



Shell with both valves free, elongated, irregular, inequi- 

 valve, widely gaping. Hinge simple, linear, edentulous; 

 ligament external, weak. Hinder muscular impression nearly 

 central, with a pedal scar in front ; pallial line with a deep, 

 angular sinus. 



