CLASS I 



PELECYPODA 



491 



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

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

 tinuous to the heaJcs ; complete armature of the hinge ivith an, inconstant posterior lateral 

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



PQ,„.. , 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. Tlie Paleozoic and many Mesozoic species so referred must be separated from 

 Isocardia. 



Isocardia Lam. (Figs. 805, 806). Inflated, smooth oi' concentrically striated ; 

 beaks distant, much produced, prosogyrate. Jura to Recent. 



B 



Viv,. S(».5. 

 IsocariUu striata d'Orb. Portlancliaii ; Cirey, Haute-Mame (after Loriol). 



Meiocardia Adams. Keeled, concentrically ribbed. Tertiary and Recent. 

 ? Glisocohos Galjli. Cretaceous ; North America. 



Fig. S06. 

 Hinge of Isocardia lunulata Nyst. Crag ; Antwerp. 



Family 30. Vesicomyacidae Dall. 



Shell as tn the Isocardiidae, hut with low and inconspiciwus beaks, the valves more 



elongated, and the lunule delimited by a sharp groove; cardinal formula ^pTaTqTa" 

 Tertiary and Recent. 



Gallogonia Dall. Pallial line deeply sinuated, and a distinct anterior lateral close 

 to the cardinal teeth. Recent ; abyssal. 



Vesicomya Dall. {Gallocardia auct., non Adams). Rounded or Tapetiform, com- 

 pressed, or not inflated, with low, inconspicuous, non-gyrate beaks. Eocene and Recent. 



D. Teleodonta. 



The most perfected type of modern teeth, to ivhich, in addition to the typical (10101) 

 cardinal series of the ordinary Teleodesmacea, there is added in the most specialised types 

 {Veneridae, Mactridae) either a roughened area [Venus), a series of extra cardinals 

 (Tivela), or accessory lamellae (Mactra), rendering the hinge more efficient, or complicated. 



