454 MANUAL OF THE MOLLUSCA. 
Sub-genera. Hemicardium (Cardissa} Cuvier. C. hemicardium, 
Pl. XIX.. Fig. 3. Shell depressed, posterior slope flat, valves 
prominently keeled. 
Lithocardium aviculare, Pl. XViil., Fig. 17. Shell triangular, 
keeled; anterior side very short; hingo-teeth 1.2, directed 
backwards; posterior laterais 2.1; anterior muscular pit minute, 
posterior imprsssion large, remote from the hinge. L. cymbulare, 
Lam,. exhibits slight indications of a byssal sinus in the front 
margins of the valves. Sossil, Hocexe, France. These shells 
present considerable resemblance te 7'riducna. 
Serripes (groenlandicus) Beck. Hinge edentulous. Arctic 
Seas, from C. Parry to Sea of Kara; fossil in the Norwich Crag. 
Fig. 253. C. leviusculum, Eichw. (after Middendorff). 
Adacna, Hichwald. C. edentulum, Pl. XIX., Fig. 4. (Acardo, 
Sw. not Brug. Pholadomya, Ag.and Mid. not Sby.) Shell coin- 
pressed, gaping behind, thin, nearly edentulous; pallial line 
sinuated. Animal with the foot (f/) compressed; siphons (s) 
elongated, united nearly to theend, plain. Distridution, 8 species. 
Aral, Caspian, Azof, Black Sea, and the embouchures of the 
Wolga, Dniester, Dnieper, and Don; burrowing in mud. C. 
Caspicum (Monodacna, Hichw.) has a single hinge-tooth, and 
C. trigonoides (Didacna, E.) rudiments of two teeth. The 
siphonal inflection varies in amount. 
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. Hillanwn, Sby. {Protocardium, Beyr.), is the type of a 
small group iu which the sides are concentrically furrowed, the 
posterior slope radiately striated; the pallial ine is slightly 
sinuated. Jura — Chalk; Europe, India. 
CoNOCARDIUM, Bronn. 
Synonyms, Lychas, Stein. Pleurorhynchus, Ph. Lurwo- 
eardium, Miinster. 
