CLASS I 



PELECYPODA 



487 



Sphaera Sow. {Palacocorhis Cour.). Lower Cretaceous. Sphaeriola Stol. Trias 



to Cretaceous. Finibriella Stol. Chalk of Britain. Corbicella Mor. and Lye. Jura. 



Mutiella Stol. (Fig. 796). Anterior cardinal Ijorder corrugated, upturned ; 



posterior rectilinear, horizontal, with a feeble lateral tooth. Upper Cretaceous. 



Family 19. Diplodontidae Dall. 



Shell suh-circular in outline, rarely nestling and irregular ; hinge with the laterals 

 obscure or absent, and the valve margins plain ; the adductor scars continuous peripher- 

 ally with the pallial line ; soft parts like the Lucinidae, but with the external limb of the 

 gills developed, and, the anal foramen not tubular. (?) Jura, Cretaceous to Eecent. 



Diplodonta Bronn (Fig. 797). Thin-shelled, orbicular, convex, concentrically 

 striate or pustulose ; cardinals 2 : 2, the left anterior and right posterior Infid ; 

 laterals obscure or absent. Tertiary and Recent. 



Ungulina Daudin. Nestling and often ir- 

 regular. Felania Recluz. Shell compressed ; 

 feeble laterals present. Axinopsis Sars ; Sphae- 

 rella and Tenea Conrad. Cretaceous to Recent. 



Family 20. Cyrenellidae Fischer. 



Fio. 797. 



Dlplodo7ita dilatata Pliill. Pliocene ; 

 Rhodes. Vi- 



Shell as in Diplodonta, but with a con- 

 spicuous epidermis ; ijallial area smooth ; pallial 

 line not sinuate ; hing'e without lateral laminae ; the cardinals like Diplodonta, or 

 with two cardinals in each valve soldered to each other dorsally ; cardinal formula 



RTivimn' ^^^'^ anterior left cardinal usually obsolete. Pliocene to Recent, in fresh and 



brackish water. 



Cyrenoida Joannis {Gyrenella Desli.). Pliocene of Florida, Recent in the Antilles 

 and West Africa. 



Joannisiella Dall. Hinge as in Diplodonta, resilixtm immersed, the larger cardinal 



bifid, the teeth not soldered above. Cardinal formula ^^^^-^^ ■ Recent ; Philippines. 



Shell 



A 



Family 21. Thyasiridae Dall. 



substance earthy, with inconspicuous epidermis and prosocoelous beaks ; valves 

 n equal, free, closed, ivith plain margins, smooth, or with feeble 



concentric striae, and usually with a radial posterior flexure ; 

 adductors Lttcinoid, pallial area often punctate ; ligament and 

 resilium parivincular, opisthodetic, sub - external, seated in a 

 groove ; area impressed ; hinge feeble vnthout lateral laminae, 

 edentulous, or with an obsolete cctrdinal tooth in the right valve. 

 Cretaceous to Recent. 



Fic. 70S. 



A, Thj/asira Hnuosa (Don.). 

 Miocene; Grand, near Vienna, 

 i/x. B, T. unicarlnata Nyst. 

 Oligocene (Septaria - clay) ; 

 Preienwalde, near Berlin. 



Thyasira Leach (Gryptodon Turton ; Axinus Sow.; Gon- 

 chocele Gabb) (Fig. 798). Smooth, thin-shelled, living in 

 deep water. Cretaceous to Recent. 



Philis Fisch. Lunule deeply indented, projecting spoon- 

 like into the cavity of the valves. Recent. 



Superfamily 8. LEPTONACEA Dall. 



Tlie inc7irrent and excurrent openings between the mantle lobes at opposite ends of the 

 body, the former a.nterior. 



