ORDER in TELEODESMACEA 413 



Family ^7. Adacnidae. DalL 



Like the Cardiidae, but thin-shelled, ivith long united siphons, a short compreszed 

 foot, a pallial sinus and obsolete 

 hinge armature, living in 

 brackish or fresh water. Ter- 

 tiary and Recent. 



Adacna, Eichw. Shell 

 elongate oval, truncate behind, 

 gaping at both extremities ; 

 siphons very Ion-;, pallial ,-inns 



deep. Miocene and Recent in 



<<, , ; ,,, <Soo LimauHiutitom conjungeru, 1'artsdi. Pliocene (Congeria - 



Caspian Sea. Brunn, near Vienna. 1/1- 



Limnocardium, Stol. (Fig. 



755). Cardinals weak, laterals strong, distant, pallial sinus moderate, shell Ho-eil 

 anteriorly. In brackish Miocene 'beds, especially the Sarmatir and Pontic horizons 

 of Eastern Europe, and in estuaries of the Aral, Black, and Caspian Seas. 



Sub-genera: Prosodacna, Tourn. (Psilodon, Cob.); Monodacna, Eichw. ; Uniocardium, 

 Capell. ; Arcicardium, Fischer. 



Superfamily 10. TRIDACNACEA. Menke. 



Soft parts rotated forward nearly 180 with relation to the valves as compared with 

 normal 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, with 

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

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

 single posterior lateral in the left, and two in the right valve. Eocene to Remit. 



Byssocardium, Mun.-Chalm., and Lithocardium, Woodw., of the Eocene, are per- 

 haps 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 tubulate, with a marked 

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

 minent prosogyrous beaks ; adductor scars sub-equal ; pedal scar adjacent ; area not dis- 

 tinctly limited ; ligament and resilium external, parivincular, set in a deep groove, con- 

 tinuous to the beaks ; complete armature of the hinge with an inconstant posterior lateral 

 in each valve, and rarely, an anterior lateral close to the cardinals; cardinal formula 



, the teeth lamelliform, and very variable in details of form. Jura to Recent. 

 B.0101 



Many species have been referred to this group solely on account of their having gyrate 

 beaks. The Palaeozoic and many Mesozoic species so referred must be separated from 

 Isocardla. 



