Fig. 308.—Cardium (Hemicardium) cardissa L., East Indies.
Fam. 2. Lunulicardiidae.—Shell equivalve, very inequilateral subtriangular, anterior margin short or truncated, with a deep lunule. Single genus, Lunulicardium (Palaeozoic).
Fam. 3. Tridacnidae.—Mantle orifices widely separated, foot short, byssiferous, 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 Once caught between their gaping valves, a man’s hand or foot can scarcely be withdrawn.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, irregularly inequivalve, umbones spiral, ligament external, cardinal teeth often a mere ridge, anterior lateral strong, nearly central, no pallial sinus. Jurassic——. Genera: Chama; Diceras (Jurassic), attached by one umbo, umbones very prominent, teeth strong; Heterodiceras (Jurassic), Requienia (Cretaceous), left valve widely spiral, attached by the umbo, right valve small, fitting on the other as an operculum, teeth obsolete; Toucasia, Apricardia, Matheronia (all Secondary strata).
Fig. 309.—A, Requienia ammonea Goldf., Neocomian, × ½; B, Hippurites cornu-vaccinum Goldf., Cretaceous, × ¼. a, right valve; f, point of fixture. (From Zittel.)
The four succeeding families require special study in a work on Palaeontology.
Fam. 5. Monopleuridae.—Shell very inequivalve, left valve operculiform, right conical or spiral, fixed at the apex, ligament prolonged in external grooves. Cretaceous——. Genera: Monopleura, Valletia.
Fam. 6. Caprinidae.—Shell very inequivalve, thick, free or fixed by apex of right valve, which is spiral or conical, left valve spiral or not, often perforated by radial canals from the umbo to the free margin. Neocomian and Cretaceous——. Principal genera: Plagioptychus, Caprina, Ichthyosarcolites, Caprotina, Polyconites.