MOLLUSCA : LAMELLIBRANCHIATA. 285 



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

 siphonal orifices surrounded by a thickened pallial border. Foot 

 finger-like and byssiferous. 111. Gen. Tridacna. 



Fam. 10. Cardiadce. Shell equivalve, heart-shaped, with radiating 

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

 open in front, siphons usually very short ; foot large, sickle - 

 shaped. 111. Gen. Cardium, Hemicardium^ Conocardium, 



Fam. II. Ludnida. Shell orbicular, and free; hinge-teeth I 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. Ludna, Diplodonta, Kellia. 



Fam. 12. Cydadidcz. 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. Cyprinida. 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 byssiferousj. 

 111. Gen. Venus, Cytherea, Vcnerupis. 



Fam. 15. Mactridtz. 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. Mactra, 

 Lutraria. 



Fam. 16. Tellinidce. 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. Tellina, Psammobia, 

 Donax. 



Fam. 17. Solenida. Shell elongated, gaping at both 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. 1 8. Myadda. Shell gaping posteriorly. Mantle almost 

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

 very small. 111. Gen. Afya, Panopcea, Glycimeris. 



Fam. 19. Anatinidce. Shell often inequivalve, with an external 

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

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

 Myochama. 



Fam. 20. GasirochanicUe. Shell equivalve, gaping, with thin eden- 

 tulous valves, sometimes cemented to a calcareous tube. Mantle- 

 margins thick in front, united, with a small pedal aperture. 



