' 
208 TRIDACNID. 
ConcHopon, Stoppani, 1865. 
Etym.—Conchos, a shell, and odos, a tooth. 
Type, C. infraliasicus, 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; two 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. (exvii, 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 cavity. Left valve with 
a compressed cardinal tooth, corresponding to a socket in the 
right valve; valves furrowed by ligamental grooves. Ligament 
external. 
Famity TRIDACNID A. 
Shell regular, equivalve, truncated in front ; ligament external; 
valves strongly ribbed, margins toothed ; muscular impressions 
blended, subcentral, obscure. 
Animal attached by a byssus, 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 conspicuously orooved : palpi very 
slender, pointed. 
The shell of Tridacna is extremely hard, being calcified until 
almost every trace of organic structure is obliterated. 
Tripacna, Bruguiére, 1789. 
Etym.—Tri, three ; dakno, to bite; a kind of oyster. (Pliny.) 
Clam-shell. 
Syn.—Chametracheea, Klein, 1753. 
Distr.—1 sp. Indian Ocean, China Seas, Pacific. Fossil, 7’. 
media. Miocene; Poland. Tridacna and Hippopus are found in 
the raised coral-reefs of Torres Straits. (Macgillivray.) 7. 
squamosa, Lam. (cxxviii, 86-88). TZ. crocea, Lam. (exxviii, 91). 
