GASTROCH^NID^. 117 



etc., and position of the hinge-ligament, give some of the prin- 

 cipal characters for genera and higher groups ; they are, as a 

 rule, and especiall}^ for recent shells, rather more satisfactory 

 than the characters used for univalve mollusca. 



Order Siphonida. Animal with siphons, and mantle-margin 

 more or less closed. 



Order Asiphonida. No siphons ; mantle-margins open. 



Order SIPHONIDA. 



Comprises most of the marine bivalve mollusca, including a 

 large i^ortion of the old order Dimyaria — having two well- 

 developed muscular impressions. 



Suborder Sinupalltata. Siphons long, partl^'^ or wholly 

 retractile ; the pallial impression upon the inside of the valve 

 having a sin as. 



Suborder Integripalliata. Siphons short, not retractile ; 

 pallial impression simple, without sinus. 



Suborder 8INUPALLIA TA. 



(Pholadacea.) 

 Family GASTROCH^NIDiE. 



Shell equivalve, gaping ; valves thin, edentuhous, united by a 

 thin, external ligament, sometimes cemented to a shelly tube 

 when adult ; adductor impressions 2, pallial line sinuated. 



Animal elongated, truncated in front, produced behind into 

 two very long, united, contractile siphons, with cirrated orifices; 

 mantle-margins very thick in front, united, leaving a small 

 opening for the flnger-like foot ; gills narrow, prolonged into the 

 branchial siphon. 



The shell-fish of this family, the Tubicolidai of Lamarck, are 

 burrowers in mud or stone. They are often gregarious, living 

 in myriads near low-water line, but are extracted from their 

 abodes with difficulty. 



Subfamily ASPEBGILLIiV^. 



Shell with both valves imbedded in the walls of a tube, with 

 their umbones visible externally. Base of the tube ornamented 

 with radiated tubuli, containing tentacular processes originating 

 in the animal's mantle. 



AsPERGiLLUM, Lam., 1818. 

 Watering-pot shell. 



Syn. — Clepsydra, Schum., 1817. Brechites, Guett., 1774, 

 Aquaria, Perry, 1811, 



