208 TRIDACNID^. 



CoNCHODON, Stoppani, 1865. 



Etym. — Covchos, a shell, and odos, a tooth. 



Type, C. inf'raliasicus^ Stop. Lower Lias ; Lombardy. 



Shell equivalve, symmetrical, very thick, cordiform, closed; 

 beaks large, angulated, involute. Ligament internal, very long, 

 marginal, attached to the posterior half of the hinge-plate. 

 Hinge massive ; in the right valve, one large rounded tooth in 

 front (placed above a dental pit), and two transverse cardinal 

 teeth ; left valve with a large circular socket, bounded below by 

 a curved lamellar tooth ; tw^o transverse and one curved teeth 

 beneath the umbo. 



DiCEROCARDiuM, Stoppani, 1865. 



Etym. — Diceras., having two horns, and cardium. 



Distr. — Fossil, 4 sp. Upper Trias; Lombardy, Northwest 

 Himalayas. D. Jani, Stop, (cxvii, 20, 21). 



Shell equivalve, symmetrical, closed, free ; umbones very 

 prominent, elongated, or spiral. Hinge-plate broad, thick, 

 separated by an interval of varying width from the edge of the 

 valve, and prolonged into the umbonal cavit3\ Left valve with 

 a compressed cardinal tooth, corresponding to a socket in the 

 right valve ; valves furrow'ed \)j ligamental grooves. Ligament 

 external. 



Family TRIDACNID^. 



Shell regular, equivalve, truncated in front ; ligament external; 

 valves strong! 3' ribbed, margins toothed ; muscular imj)ressions 

 blended, subcentral, obscure. 



Animal attached b}^ a b}' ssus, or free ; mantle-lobe extensively 

 united ; pedal opening large, anterior ; siphonal orifices sur- 

 rounded by a thickened pallial border ; branchial plain ; anal 

 remote, with a tubular valve ; shell-muscle single, large and 

 round, with a smaller pedal muscle close to it behind ; foot 

 finger-like, with a byssal groove ; gills two on each side, narrow, 

 strongly plaited, the outer pair composed of a single lamina, the 

 inner thick, with margins conspicuousl}^ grooved ; palpi very 

 slender, pointed. 



The shell of Tridacna is extremely hard, being calcified until 

 almost every trace of organic structure is obliterated. 



Tridacna, Bruguiere, 1189. 



Etym. — TVi, three ; dakno, to bite ; a kind of oyster. (Pliny.) 

 Clam-shell. 



Syn. — Chametrachfea, Klein, 1153. 



Distr. — 1 sp. Indian Ocean, China Seas, Pacific. Fossil, T. 

 media. Miocene; Poland. Tridacna and Hippopus are found in 

 the, raised coral-reefs of Torres Straits. ( Macgilli vra3\ ) T. 

 squamosa, Lam. (cxxviii, 86-88). T. crocea, Lam. (cxxviii, 91), 



