EULAMELLIBRANCHIATA— CARDIACEA 



455 



byssus ; shell equivalve, more or less racliately ribbed, hinge with 

 one or two cardinals in each valve, laterals variable, ligament ex- 

 ternal, two adductors. Brackish water or marine. Devonian -. 



Genera : Byssocardium and Lithocardium (Tertiary), Cono- 

 cardmm (Palaeozoic), Cardium (with 

 many sections, including Hemicar- 

 diuiii), Limnocardium (subg. Didacna, 

 Monodacna, Adacna). 



Fam. 2. Lunulicardiidac. — Shell 

 equivalve, very inequilateral subtri- 

 angular, anterior margin short or trun- 

 cated, with a deep lunule. Single genus, 

 Lunulicardium (Palaeozoic). 



Fam. 3. Tridacnidae. — Mantle ori- 

 fices widely separated, foot short, bys- 

 siferous, no anterior adductor ; shell 

 equivalve, large, thick, usually gaping 

 in front, one cardinal tooth and one or 

 each valve, no pallial sinus. Miocene— 

 Hippopus. The muscular power of the great Tridoxna is immense. 

 Once caught between their gaping valves, a man's hand or foot 

 can scarcely be withdrawn. Two valves of 2\ gigns in the British 

 Museum weigh respectively 154 and 156 lbs. 



FA:\r. 4. Chamidae. — Mantle orifices widely separated, foot 

 short, no byssus, Ijoth adductors present, ovary invading the 



mantle lobes ; shell fixed, irregu- 

 larly inequivalve, uml:)ones spiral, 

 ligament external, cardinal teeth 

 often a mere ridge, anterior lateral- 

 strong, nearly central, no pallial 

 sinus. Jurassic . Genera : 



Fig. 308. — Cardium (Hemicar- 

 dium) cardissa L., East ludies. 



two posterior laterals in 

 — . Genera : Tridacna, 



Chama; Diceras (Jurassic), attached 

 by one umbo, umbones very promi- 

 nent, teeth strong ; Hcterodiccras 

 (Jurassic), Rcquienia (Cretaceous), 



A - B 



Fig. 309. — A, Requienia ammonea Goldf., 



Neocomian, x ^ ; B, iiipimrites h'ft valve widely Spiral, attached 



coniu-vaccimim Goldf., Cretaceous, i , i •, • i , i , „, ,11 



xi. «, right valve; /.point of fix- ^O' tlie umbo, right valve small, 



ture. (From Zittel.) 



teeth 



culum, teeth obsolete ; 

 Secondary strata). 



fitting on the other as an oper- 

 Toucasia, Apricardia, 3Iatheronia (all 



