XVI 



EULAMELLIBRANCHIATA — CARDI ACEA 



455 



Fig. 308. — Cardium (Hemicar- 

 dium) cardissa L., East Indies. 



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

 external, two adductors. Brackish water or marine. Devo- 

 nian . Genera: B i/sso car dium eind Lithoeardium (Tertiary), 



Conocardium (Palaeozoic), Cardium 

 (with many sections, including Remi- 

 cardium'), Limnocardium (subg. Bi- 

 dacna, Monodacna, Adacna). 



Fajvi. 2. Lunulicardiidae. — 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 two posterior laterals in 



each valve, no pallial sinus. Miocene . Genera : Tridacna, 



Hippopus, The muscular power of the great Tridacna is immense. 

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

 can scarcely be withdrawn. Two valves of T. gigas in the 

 British Museum weigh respectively 154 and 156 lbs. 



Fam. 4. Chamidae. - — Mantle orifices widely separated, foot 

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



mantle lobes ; shell fixed, irregu- 

 larly inequivalve, umbones spiral, 

 ligament external, cardinal teeth 

 often a mere ridge, anterior lateral 

 strong, nearly central, no pallial 



sinus. Jurassic . Genera : 



Qhama ; Dicer as (Jurassic), at- 

 tached by one umbo, umbones 

 very prominent, teeth strong; 

 Heterodiceras (Jurassic), Re- 



ammonea 



A 



Fig. 309. — A, Requienia 



Goidf., Neocomian, x | ; B, Hippu- quienia (Cretaccous), left valve 



Htes cornu-vaccinum Goldf., Creta- ^^-^^^y ^^-^^^^ attached 



ceous, X \. a, right valve; /, point 



by the 

 of fixture. (From zittei.) umbo, right valve small, fitting on 



the other as an operculum, teeth obsolete; Toucasia^ Apricardia, 

 Mather onia (all Secondary strata). 



