GASTROCHANIDA. ia bey 
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 especially for recent shells, rather more satisfactory 
than the characters used for univalve mollusca. 
Order SipHontpa. Animal with siphons, and mantle-margin 
more or less closed. 
Order AsipHonipA. No siphons; mantle-margins open. 
OrpveR SIPHONIDA. 
Comprises most of the marine bivalve mollusea, including a 
large portion of the old order Dimyaria—having two well- 
developed muscular impressions. 
Suborder Sryupatmiata. Siphons long, partly or wholly 
retractile; the pallial impression upon the inside of the valve 
having a sinus. 
Suborder INTEGRIPALLIATA. Siphons short, not retractile ; 
pallial impression simple, without sinus. 
Suborder SINUPALLIA TA. 
(Pholadacea.) 
Famity GASTROCH HZ NIDZ. 
Shell equivalve, gaping; valves thin, edentulous, 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 finger-like foot; gills narrow, prolonged into the 
branchial siphon. 
The shell-fish of this family. the Tubicolide 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. 
SuspraMiILty ASPHRGILLINA. 
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. 
