202 STRUCTURE OF THE MOLLUSCA. 



generally ribbed and laminated. Tridacna, Chama, Iso- 

 cardia. 



The Cycladina have the mantle-lobes free beneath, united 

 behind, and forming an internally double tube, the foot 

 very extensile, cylindrical ; the shell very thin and delicate. 

 They reside in fresh water. Cyrena, Cyclas, Pisidium. 



The Venerina have the mantle-lobes free beneath, united 

 behind to form a tube containing two siphons, the foot com- 

 pressed, extensile; the shell varying in form, convex, con- 

 centrically striated, with prominent umbones, and the hinge 

 with from two to four divergent teeth, and elongated lateral 

 teeth. Lucina, Cyprina, Astarte, Venus, Cytherea, Vene- 

 rupis. 



The Cardiina have the mantle closed, with an opening 

 for the foot, and two extensile tubes, the foot large, com- 

 pressed, tapering ; the shell equivalve, entirely closed, gen- 

 3rally very convex, the hinge with central and lateral teeth. 

 Cardium, Donax. 



The TelUnina have the mantle open anteriorly for the 

 foot, and bordered with tentacular appendages, closed be- 

 hind, but with an aperture for the elongated and separated 

 siphons; the foot much compressed, sharp-edged; the shell 

 various in form, compressed, inequivalve, inequilateral, the 

 posterior end shorter and flexuous ; the hinge with one or 

 two small central teeth. Tellina, Cryptodon. 



The Solenina have the animal much elongated, the mantle 

 closed, but with an opening in front for the foot, and form- 

 ing behind a tube, internally double; the foot conical; the 

 shell long, equivalve, extremely inequilateral, thni, gaping 

 at both ends ; the hinge with not more than two small pro- 

 minent teeth in each valve. Solen, Psammobia. 



The Mactrina have the animal oval or elliptical, with the 

 mantle closed, but open in front for the foot, and forming 

 behind a tube internally double; the foot large, compressed, 

 bent; the shell equivalve, inequilateral, more or less gaping 

 at both ends ; the hinge with an oblique triangular depres- 

 sion for the internal ligament, there being also a small 

 external ligament. Mactra, Lutraria, Amphidesmoy Anatina. 



