410 



MOLLUSCA 



SUB-KINGDOM VI 



Diplodonta, Bronn (Fig. 748). Thin-shelled, orbicular, convex, concentrically 

 striate or pustulose ; cardinals 2. 2, the left anterior and right posterior bifid ; laterals 



obscure or absent. Tertiary and Recent. 



Ungulina, Daudin. Nestling and often 

 irregular. Felania, Recluz. Shell compressed ; 

 feeble laterals present. Axinopsis, Sars ; Sphae- 

 rella, Tenea, Conrad. Cretaceous to Eecent. 



FIG. T48. 

 Diplodonta dilutata, Phill. 



Family 20. Cyrenellidae. Fischer. 



Pliocene ; 



Rhodes. 1/1- Shell as in Diplodonta, but with a con- 



spicuous epidermis; pallial area smooth; pailial 



line not sinuate; hinge without lateral laminae; the cardinals like Diplodonta, 

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



= , the anterior left cardinal usually obsolete. Pliocene to Recent, in fresh or 



brackish water. 



Cyrenoidea, Joannis (Gyrenella, Desh.) Pliocene of Florida, Recent in the Antilles 

 and West Africa. 



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



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



Family 21. Cryptodontidae. Dall. 



Shell substance earthy, with inconspicuous epidermis and 

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

 concentric striae, and usually with a radial posterior flexure ; 

 adductors Lucinoid, pallial area often punctate ; ligament and 

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

 groove ; area impressed ; hinge feeble without lateral laminae, 

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

 Cretaceous to Recent. 



Cryptodon, Turton (? Axinus, Sow. ; Conchocele, Gabb), 

 (Fig. 749). 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. 



ous beaks; valvt 



n 



A, Cryptodon sinuosus, Don. 

 Miocene ; Grund, near Vienna. 

 Vl- B, C. imicarinatits, Nyst. 

 Oligocene (Septaria - clay) ; 

 FreiemvaMe, near Berlin. 



Superfamily 8. LEPTONACEA. Dall. 



The incurrent and excurrent openings between the mantle lobes at opposite ends of the 

 body, the former anterior. 



This group contains a great many commensal, nestling, or parasitic forms ; if indepen- 

 dent usually very active, crawling like Gastropods on a sub-reptary foot, and with the mantle 

 edges more or less reflected over the valves. 



Family 22. Leptonidae. Dall. 



Shell cellulo-crystalline with a periostracum ; valves equal, free, smooth-edged, often 

 gaping, variably sculptured ; adductor scars peripheral, sub-equal ; pallial line simple ; 

 area obscure or none; ligament parivincular, opisthodetic, external, often obsolete; 

 resilium usually internal, sub-umbonal or oblique; hinge plate narrow, channelled to 

 receive the resilium; hinge variable, normally consisting of one or two radiating 



