461 



MANUAL OF THE MOLLITSCA. 



8uh-genera. Hemicardium (Cardissa) Cuyier. C. L.emicai'dium, 

 PL XIX.. Fig. 3. Shell depressed, posterior slope flat, valves 

 prominently keeled. 



Lithocardium aviculare, PL XVIII., Fig. 17. Shell triangular, 

 keeled; anterior side very sliort; hinge-teeth 1.2, directed 

 backwards ; posterior laterals 2.1; anterior muscular pit minute, 

 posterior impression large, remote from the hinge. L. cymhulare, 

 Lam,, exhibits slight indications of a byssal sinus in the front 

 margins of the valves. Fossil, Eocene, France. These shells 

 present considerable resemblance to Tridacna. 



Serrijpes (groenlandicus) Beck. Hinge edentulous. Arctic 

 Seas, from 0. Parry to Sea of Kara ; fossil in the Norwich Crag. 



Fig. 253. C. IcEvmsculum, Eichw. (after Middendorff). 



Adacna, Eichwald. C. edentulum, PL XIX., Fig. 4. (Acardo, 

 Sw. not Brug. Pholadomya, Ag. and Mid. not Sby.) Shell com- 

 pressed, gaping behind, thin, nearly edentulous; pallial line 

 sinuated. Animal with the foot (/) compressed; siphons (s) 

 elongated, united nearly to the end, plain. Distrihution, 8 species. 

 Aral, Caspian, Azof, Black Sea, and the embouchures of the 

 Wolga, Dniester, Dnieper, and Don; burrowing in mud. (7. 

 Caspicum (Monodacna, Eichw.) has a single hinge-tooth, and 

 G. trigonoides (Didacna, E.) rudiments of two teeth. The 

 siphonal inflection varies in amotint. 



Distribution, 200 species. World-wide ; from the sea-shore to 

 140 fathoms. Gregarious on sands and sandy mud. 

 ' Fossil, 330 species. Upper Silurian — . Patagonia — 

 Southern India. 



C. Hillanum, Sby. (Protocardium, Beyr.), is the type of a 

 small group in which the sides are concentrically furrowed, the 

 posterior slope radiately striated; the pallial line is slightly 

 sinuated. Jura — Chalk ; Europe, India. 



Synonyms, Lychas, Stein, 

 cardium, Miinster. 



CoNOCARDlXJM, Bronn. 



PleurorhynchuSj Ph. Lunulo- 



