MOLLUSCA: LAMELLIBRANCHIATA. 307 



Fam.<j. Tridacnida. — Shell equivalve ; ligament external; muscular 

 impressions blended, sub-central. Animal attached by a byssus, 

 or free. Mantle-lobes extensively united ; pedal aperture large ; 

 siphonal orifices surrounded by a thickened pallial border. Foot 

 finger-like and byssiferous. IIL Gen. Tridacna. 



Fam. 10. Cardiada. — Shell equivalve, heart-shaped, with radiating 

 ribs ; cardinal teeth 2 ; lateral teeth i-i, in each valve. Mantle 

 open in front, siphons u.sually very short; foot large, sickle- 

 shaped. III. Gen. Cardium, Hemicardium, Conocardium. 



Fum. II. Lucinidie. — Shell orbicular, and free; hinge-teeth 1 or 2; 

 lateral teeth i-i, or obsolete. Mantle-lobes open below, with 

 one or two siphonal orifices behind ; foot elongated, cylindrical, 

 or strap-shaped. 111. Gen. Lztcina, Diplodonta, Kellia. 



Fam. 12. CycladidiB. — Shell sub-orbicular, closed ; hinge with 

 cardinal and lateral teeth ; ligament external. Mantle open in 

 front; 1-2 siphons, more or less united. Foot large, tongue- 

 shaped. 111. Gen. Cyclas, Cyrena. 



Fam. 13. CyprinidtB. — Shell equivalve, closed ; ligament external ; 

 cardinal teeth 1-3 in each valve, and usually a posterior tooth. 

 Mantle-lobes united behind by a curtain pierced with two siphonal 

 orifices. Foot thick, and tongue-shaped. 111. Gen. Cyprina, 

 Astarte, Isocardia. 



Subdivision II. Sinu-pallialia. — Respiratory siphons large; pallial 

 line sinuated. 



Fam. 14. VeneridcE. — Shell regular, sub-orbicular or oblong ; liga- 

 ment external ; hinge with usually 3 diverging teeth in each valve. 

 Animal usually free and locomotive ; mantle with a rather large 

 anterior opening ; siphons unequal, more or less united. Foot 

 tongue'shaped, compressed, sometimes grooved and byssiferous. 

 111. Gen. Venus, Cytherea, Venerupis. 



Fam. 15. Macirida. — Shell equivalve, trigonal ; hinge with two 

 diverging cardinal teeth, and usually with anterior and posterior 

 lateral teeth. Mantle more or less open in front ; siphons united, 

 with fringed orifices ; foot compressed. 111. Gen. Madra, 

 Lutraria. 



Fam. 16. Tellinidcs. — Shell free, usually equivalve and closed ; 

 cardinal teeth 2 at most, laterals I-I, sometimes wanting. Liga- 

 ment on the shortest side of the shell, sometimes internal. 

 Mantle widely open in front Siphons separate, long and slender ; 

 foot tongue-shaped, compressed. 111. Gen. Tdlina, Psammobia, 

 Donax. 



Fam. 17. Solenidce. — Shell elongated, gapingatboth ends; ligament 

 external;, hinge-teeth usually 2—3. Siphons short and united 

 (in the long-shelled genera), or longer and partly separate (in the 

 genera with shorter shells). Foot very large and powerful. Gills 

 prolonged into the branchial siphon. 111. Gen. Solen, Cultellus, 

 Solecurtus. 



Fam. 18. Myacida. — Shell gaping posteriorly. Mantle almost 

 entirely closed ; siphons united, partly or wholly retractile. Foot 

 very small. 111. Gen. Mya, Panopaa, Clycimeris. 



Fam. 19. Anatinidcs. — Shell often inequivalve, with an external 

 ligament. Mantle-lobes more or less united ; siphons long, more 

 or less united. Foot small. 111. Gen. Aiiatina, Pholadomya, 



