550 TRANSACTIONS OF THE WAGNER FREE 



ous, the pedal retractors attached to the shell behind the adductor; mantle 

 edges papillose, united except for the siphonal and pedal foramina; pallial line 

 simple ; anal and branchial foramina ventrad of the beaks, distant from each 

 other, not tubular, the anal with a valve ; pedal foramen directly in front of the 

 beaks, apparently (but not morphologically) dorsal ; dioecious, marine. 



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

 free, with a byssal gape, radially sculptured, with serrate margins and proso- 

 cceIous beaks ; ligament and resilium as in Cardiidce ; 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 fauna. 

 Ex. Tridacna, Hippopus, Byssocardium, Lithocardium. 



Neumayr confused the large pedal muscles of Tridacna with the adduct- 

 ors, and erroneously concluded that there were two adductors ; though it is 

 probable that the pedal muscles, which are strongly knit together, assist to 

 some extent in closing the valves. In Tridacna, which is practically sessile 

 by its large byssus, the foot is obsolete and the pedo-byssal scars enormous. 

 In Hippop2is, which is free, the foot is larger and the pedal scar inconspicuous, 

 showing how much special function modifies such characters. 



Superfamily ISOCARDIACEA. 



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 ISOCARDIID^. 



Gills as in Cardium, palpi small, not united di.stally ; foot short, keeled, 

 compressed, grooved, with a byssal apparatus ; pallial line simple ; anal fora- 

 men without a conspicuous valve, branchial larger, complete, both papillifer- 

 ous, not produced into siphons ; dioecious, marine in shallow water. 



Shell substance cellulo-crystalline, the external layer not tubulate, with 

 a marked epidermis ; valves equal, free, rotund, completely closed, with plain 

 margins and prominent prosogyrous beaks; adductor scars subequal ; pedal 

 scar adjacent; area not distinctly limited; ligament and resilium external, 

 parivincular, set in a deep groove, continuous to the beaks ; the complete 

 armature of the hinge comprises an inconstant posterior lateral in each valve, 

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

 E^^TTI' the teeth lamelliform and very variable in details of form. 



Jurassic to recent fauna. 

 Ex. Clisocolus, Isocardia, Meiocardia, ? Vesiconiya. 



The peculiarity of the cardinal teeth is not the result of torsion following 

 the gyrate beaks, for in Callocardiidce, where the beaks are short and the liga- 



