408 
MOLLUSCA. SIPHONIDA. 
SIPHONIDA. 
I. One syphon only, formed by the union of the cloak poste- 
riorly. 
A. Shell transverse ; beaks obsolete, ligament, linear and ecc-^ 
ternal. 
a. Furnished with a byssus ; marine. Mytilid^e. 
Mytilus. 
Modiolus. 
Lithodomus. 
aa. Destitute of a byssus ; inhabiting fresh water. UNiOKiDiE., 
6. Hinge simple. 
Anodon. 
(Iridina.) 
h b. Hinge with teeth. 
Unio. 
Alismadon. 
A A. Shell with prominent beaks, the ligaments internal. Car^ 
ditadce. 
Cardita. 
Venerkardia. 
Crassatella. 
II. Cloak closed anteally and retrally, with three openings to-. 
wards the middle of the ventral margin, the first for the 
byssus, the middle one for the entrance of water, and the 
anal posterior one. (Tridacnad^.) 
III. Cloak open anteally, with two retral syphons, the one 
branchial and the other anal, which are sometimes pro- 
duced into united or separate tubes. A foot and two ad- 
ductor muscles. 
A. Shell heart-shaped ; beaks prominent, incurved, with 
strong hinge-teeth ; ligament external. 
a. Shell adhering, irregular, inequivalve ; anterior aperture 
of the cloak small, corresponding with the size of the 
foot. Chamad.®. 
Chama. 
(Diceras) 
(Etheria.) 
a a. Shell free, regular, equivalve ; foot large. Cardiada:. 
Isocardia 
Hippopodium. 
Cardium. 
Pholadomya 
(Cypricardia.) 
