408 MOLLUSCA. SIPHONIDA. 



SIPHONIDA. 



I. One syphon only, formed by the union of the cloak poste- 



riorly. 



A. Shell Iransverse ; beaks obsolete, ligament, linear and ex- 

 ternal. 



a. Furnished with a byssus ; marine. MytiliDjI:. 



Mytilus. 



Modiolus. 



Lithodomus. 



aa. Destitute of a byssus; inhabiting fresh water. Unionidje. 



h. Hinge simple. 

 Anodon. 

 (Iridina.) 



hb. Hinge with teeth. 

 Unio. 

 Alismadon. 



A A. Shell with prominerit beaks, the ligaments internal. Car- 

 ditadce. 



Cardita. 



Veiiericardia. 



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, con'esponding with the size of the 

 foot. Chamad^. 



Ctiama. 



(Diceras) 



(Etheria.) 



a a. Shell free, regular, equivalve ; foot large. Caiidiad«. 

 Isocardia 

 Hippopodium. 

 Cardium. 

 P1u>ladomya 

 (Cypricardia.) 



