490 



MOLLUSCA 



PHYLUM VI 



with prominent beaks. Represented by about 200 recent and over 100 fossil species. 

 Trias to Recent. 



A very large number of subgenera and sections have been proposed, based chiefly on the 

 external sculpture. Some of the more conspicuous groups are the following : — 



Prntocardia Beyr. (Fig. 802). Cretaceous. Discors Desh. (Fig. 803) ; Laevicardiuvi 

 Swains. ; Scrripcs Beck ; Fragum Bolten ; Pa2)yridca Swains. Tertiary and Recent. 

 Didacna Eichwald, estuarine, leads toward the next family. 



Family 27. Limnocardiidae Stoliczka. 



Like the Gardiidae, but thin-shelled, witli long united siphons, a short compressed 



foot, a pallial sinus and obsolete 

 hinge armature, living in 

 brackish or fresh water. Ter- 

 tiary and Recent. 



Adacna Eiclnv. Slicll 



elongate oval, truncate behind, 



gaping at both extremities ; 



siphons very long, pallial 



sinus deep. Miocene, and 



Recent in Caspian Sea. 



Limnocardium Stol. (Fig. 804). Cardinals weak, laterals strong, distant, pallial 



sinus moderate, shell closed anteriorly. In brackish Miocene beds, especially the 



Sarmatic and Pontic horizons of Eastern Europe, and in estuaries of the Aral, Black 



and Caspian Seas. 



Fic. 804. 



Limnocnrdium conjungens Partsch. Pliocene (Congeria Stage) ; 

 Brunn, near Vienna. Vi- 



Subgenera : Prosodacna Tourn. {Psilodon Cob.) ; Monodacna Eichw. 

 Arcicnrdiuin Fischer. 



Uniocardiu7)i Capell. ; 



Superfamily 10. TRIDACNACEA Menke. 



Soft parts rotated forward nearly 90° with relation to the valres as compared with 

 typical dimyarian Pelecypods, the anterior adductor wanting, and the posterior nearly 

 central in the shell ; cardinal teeth lamellar, oblique. 



Family 28. Tridacnidae Cuvier. 



Shell very densely porcellanous, with no visible epidermis; valves equal, free, witli 

 a byssal gape, radially sculptured, with serrate margins and prosocoelous beaks ; ligament 

 and resilium as in the Gardiidae ; hinge with a single oblique cardinal in each valve, a 

 single posterior lateral in the left, and tivo in the right valve. Eocene to Recent. 



Byssocardium Mun.-Chalni. and Lithoccordium Woodw., of the Eocene, are 

 ])er]iaps precursors of the Recent Tridacna Brug. and Hippopus Lamarck. 



Superfamily 11. ISOCARDIACEA Ball. 



Lobes of the mantle closed, except for the pedal and siphonal openings, smooth, double- 

 edged ; foot short, compressed ; sculpture of the shell faint or concentric ; cardinal teeth 

 lamellar, parallel with the hinge margin. 



Family 29. Isocardiidae Gray. 



Shell substance cellulo-crystalline, the external layer not tubuJate, with a marked 

 epidermis; valves equal, free, rotund, completely closed, with, plain, margins and jiro- 



