ORDER III TELEODESMACEA 413 
Family 27. Adacnidae. Dall. 
Like the Cardiidae, but thin-shelled, with 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, Eichw. Shell 
elongate oval, truncate behind, 
gaping at both extremities ; 
siphons very long, pallial sinus 
deep. Miocene and Recent in a hon 
Caspian Sea. Bauer Cada oe Partsch. Pliocene (Congeria Stage) ; 
Limnocardium, Stol. (Fig. 
755). Cardinals weak, laterals strong, distant, pallial sinus moderate, shell closed 
anteriorly. In brackish Miocene beds, especially the Sarmatic and Pontie horizons 
of Eastern Europe, and in estuaries of the Aral, Black, and Caspian Seas. 

Fic. 755. 
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 Cardidae ; hinge with a single oblique cardinal in each valve, a 
single posterior lateral in the left, and two in the right valve. Eocene to Recent. 
Byssocardiwm, Mun.-Chalm., and Lithocardiwm, Woodw., of the Eocene, are per- 
haps precursors of the recent Tridacna, Brug., and Hippopus, Lamarck. 
Superfamily 11. ISOCARDIACEA. Dall. 
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- 
tinetly limited ; ligament and resilivm 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 
L1010 
nae the teeth lamelliform, and very variable in details of form. Jura to Recent. 
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 
_Lsocardia. 
Al 
