328 MANUAL OF ZOOLOGY. 



Fam. 9. Tridacnidce. 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. 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. Lucinidce. 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. Lucina, Diplodonta, Kellia. 



Fam. 12. Cyclqdidce. 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. Cyclcis, Cyrena. 



Fam. 13. Cyprinidce. 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, Venerida. 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. Mactridce. 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. Tettinidce. 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, 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. Myaddce. Shell gaping posteriorly. Mantle almost 

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

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



Fam. 19.. Anatinida. 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. 



