412 MOLLUSCA SUB-KINGDOM VI 
Family 26. Cardiidae. Fischer. 
Shell substance cellulo-crystalline, with the external layer more or less tubular ; valves 
equal, free, gaping slightly behind, the beaks prosocoelous, the margins usually serrate or 
radially striated ; adductor scars sub-equal, the pedal distinct and usually distant ; 
ligament and resilium parivincular, external, short, set in a groove; area obscure ; com- 
plete hinge armature consisting of an anterior and posterior lateral in the left, and two 

Fic. 752. 
Cardium productum, Sow. Turonian; St. Gilgen, Salzburg. 1/;. 
anterior and one posterior lateral in the right valve, any or all of which may be absent ; 
cardinal formula a the teeth simple, smooth, never bifid, one cardinal in each valve 

usually persistent, the others inconstant. Trias to Recent. 
Cardium, Linn. (Figs. 752-754). Cordate, inflated, radiately ribbed or striated, 

Fic. 753. 
Protocardia bifrons, 
Reuss. Turonian; 
Strobl-Weissenbach am 
Wolfgangsee, Austria. Cardium (Discors) discrepans, Bast. Miocene; Dax, near 
1/y. Bordeaux. 1/3. 

with prominent beaks. Represented by about 200 recent and over 100 fossil species. 
Trias to Recent. 
A very large number of sub-genera and sections have been proposed, based chiefly on the 
external sculpture. Some of the more conspicuous groups are the following :— 
Protocardia, Beyr. (Fig. 753). Cretaceous. Discors, Desh. (Fig. 754); Laevicardiwm, 
Swains.; Serripes, Beck ; Hemicardiwm, Cuvier ; Papyridea, Swains. Tertiary and Recent. 
Didaena, Kichwald, estuarine, leads toward the next family. 
