CLASS I 



PELECYPODA 



475 



Family 7. Sphaeriidae Dali, 



Shell as in the Gyrenidae, but small, with a feehle, short ligament, a simple pallial 

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

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

 the Gyrenidae, distinct. Upper Cretaceous to Recent, 



Sphaerium Scop, {Gyclas Lam., non Bnig,). Brancliial siphon complete ; shell 

 equilateral, inflated, rounded. Upper Cretaceous to Recent. 



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



Gorneocydas Ferussac {Pisidium Pfeitf.). i Shell ineqiiilateral ; branchial siphon 

 merged with the pedal opening. Eocene to Recent. 



Superfamily 4. CARDITACEA Menke. 



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

 Mesozoic, forming in one sense a sort of parallel series with the Ästartidae, with ivhich it 

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

 cardinal tooth. 



Family 8. Oarditidae Gill. 



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

 the anterior adductor scar ; ligament external, parivincular ; re'silium usually included 

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

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

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



p • Marine; dioecious, frequently viviparous. Trias to Recent. 



The earlier forms approaeh the Ästartidae and Pleurophoridae so closely that they can 

 hardly be discriminated. 



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



m 



Fio. 7G3. 



Palacocardita crenata 

 (Münst.), Upper 

 Trias ; St. Cassian, 

 Tyrol. 



Fi(i. 764. 

 Venericanlia imbricata Lam. Eocene ; Grignon, near Paris. 



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

 cardinals long and oblique. Trias to Recent. 



Palacocardita Conr. (Fig. 763). Like Gardita, but with a posterior lateral 

 tooth. Trias and Cretaceous. 



Venericardia Lam. (Fig. 764). Rounded or cordate ; lateral teeth absent or 

 obsolete. Cretaceous to Recent. 



