398 



MOLLUSCA 



SUB-KINGDOM VI 



Family 7. Sphaeriidae. Dall. 



Shell as in the Cyrenidae, but small, with a feeble, short ligament, a simple pallial 

 line, and no hinge plate ; cardinal teeth usually two in each valve, variable, very thin, 

 often nearly parallel to the hinge margin or defective in part of the series ; laterals as in 

 the Cyrenidae, distinct. Upper Cretaceous to Eeceiit. 



Sphaerium, Scop. (Cyclas, Brug.) Branchial siphon complete ; shell inflated, 

 rounded. Upper Cretaceous to Recent. 



Sub-genus Eupera, Bgt. Shell compressed, trapezoid. Tertiary and Recent ; sub-tropical. 



Pisidium, Pfr. Shell inequilateral ; branchial siphon merged with the pedal 

 opening. Eocene to Recent. 



Superfamily 4. CAEDITACEA. Menke. 



This group appears to have branched off from the Astartoid radical in the early 

 Mesozoic, forming in one sense a sort of parallel series with the Astartidae, with which it 

 is contrasted most obviously by its prevailingly radial sculpture and prolonged posterior 

 cardinal tooth. 



Family 8. Carditidae. Gill. 



Shell as in the Astartidae, but usually with radial sculpture, the pedal adjacent to 

 the anterior adductor scar ; ligament external, parivincular ; resilium usually included 

 in the ligament, rarely internal; hinge fully developed, with the laminae as in the 

 Astartidae, and usually obsolete ; the anterior cardinal often obsolete, the posterior pro- 

 longed parallel with the dorsal margin even below the ligament. Full cardinal formula 



Trias to Recent. 



The earlier forms approach the Astartidae and Pleurophoridae so closely that they can 

 hardly be discriminated. 



* Gardita, Brug. Elongate, quadrate, with prominent, very anterior beaks ; sculp- 



FIG. 714. 



Pulaeocardita crenata , 

 Miinst. sp. Upper 

 Trias ; St. Cassian, 

 Tyrol. 



FIG. 715. 

 Venericardia imbricata, Lam. Eocene ; Grignon, near 



ture radial and usually imbricated, commonly with a lunule ; inner margins dentate : 

 cardinals long and oblifjue. Trias to Recent. 



Palaeocardita, Conr. (Fig. 714). Like Cardita, but with a posterior lateral 

 tooth. Trias and Cretaceous. 



Venericardia, Lam. (Fig. 7 1 5). Rounded or cordate ; lateral teeth absent or 

 obsolete. Cretaceous to Recent. 



