XVI 



EULAMELLIBRANCHIATA — CARDIACEA 



455 



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 and Litliocardium (Tertiary), 



Conocardium (Palaeozoic), Cardium 

 (with many sections, including Hemi- 

 cardiurti)^ Limnocardium (subg. Di- 

 dacna, Monodacna, Adacna). 



Fam. 2. Lunulicardiidae. — Shell 

 equivalve, very inequilateral subtri- 

 angular, anterior margin short or trun- 

 cated, with a deep lunule. Single 

 genus, Lunulieardium (Palaeozoic). 



Fam. 3. Tridacnidae, — Mantle ori- 

 fices widely separated, foot short, bys- 



. P , . 11, in Fig. 308. — Cardium (Hemicar- 



siterous, no anterior adductor; shell dium) cardissa i.., East indies. 



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 can scarcely be withdrawn.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 ; Diceras (Jurassic), at- 

 tached by one umbo, umbones 

 very prominent, teeth strong ; 

 Heterodiceras (Jurassic), Re- 

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



rites cornu-vaccinum Goldf., Greta- • i i • i j.i. t, j t_ xi. 



ceous, X |. a, right valve; /, point ^"^^^^^ly Spiral, attached by the 

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



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



Matheronia (all Secondary strata). 



Fig 



Requierda amraonea 



