ORDER I 



PRIONODESMACEA 



369 



ternalj shell structure coarse/// jiritumtic, with a tliin, jnirlit.il, nacreous lining; bynsifer- 

 ou*. Devonian to Recent. 



Palaeopinna, Hall. Devonian ; North America. 



Aviculopinna, Meek. A v.-ry >niall wing in front of the beaks. Carboniferous 

 and Permian. 



Pinna, Linn. (Fig. 620). Shell thin, with a long hinge line ; valves carinatc, 

 triangular. Jura to Recent. 



Pinnigena, Sauss. (Trichites, Plott), (Fig. 621). Muscular impression very large ; 

 prismatic layer extremely thick ; sculpture divaricate. Jura and Cretaceous. 



Atrina, Gray. Shell with broad adductor scars ; short hinge line and entire 

 nacreous layer. Carboniferous to Recent. 



Cyrtopinna, Miirch. Jura to Recent. 



t Family 19. Conocardiidae. Neumayr. 



Shell sub-trigonal, anteriorly truncate and gaping, the margins of the gape frequently 

 produced into a tube-like rostrum and sharply serrate below, the posterior end usually 

 alate, tJie wing divided internally by a longitudinal ridge ; 

 dimyarian, the anterior adductor scars smaller; equivalve 

 more or less gaping behind; Schizodont, with a single 

 anterior lateral, and an obscure or obsolete cardinal tubercle ; 

 area ill-defined, amphidetic; ligament external, parivincular ; 

 shell structure cancellate, or built up of hollow prisms resem- 

 bling those of Pinna, but not solid; valves thick, internally 

 marginate ; byssiferous (?) ; marine. Silurian to Carbon- 

 iferous. , Conocardium alaefvnne, Sow. Car- 

 boniferous Limestone : Tournay, 



This group includes Conocardium, Bronn ; (Pleuro- Belgium. Vi- 

 rhynchus, Phill.), (Fig. 622) ; and Hh^ndocardium, Fischer. 



It is extremely isolated, and comprises some fifty species. These remarkable shells have 

 been referred by most palaeontologists to the Cardiacea, with which they have no connection 

 whatever except analogy of form with a few aberrant Cardiidae. 



Family 20. Pernidae. Zittel. 



Shell sub-mytiliform, with a broad posterior wing; monomyarian, the anterior addtn-fm- 

 absent in the adult; inequivalve, teeth irregular or absent, with a serial mn.lHriiK-n.lnr 

 ligament ; byssiferous, with a moderate gape, or none. Permian to Recent. 



FIG. 622. 



Fio. 623. 

 A, Gervillia aviculoides, Sow. Oxfordian ; Dives, Calvados. ]:, <;. Uiuartt, Huvignier. Hinge. 



This family differs from the Pteriidae chiefly by its multivincular ligament in the adult 

 state. It finds its maximum development in the Jura and Cretaceous. 



Bakewellia, King. Small, obliquely elongated, alatr liehiud, three to four deii- 

 ticulations under the l>eaks. Permian. 



VOL. I 



2 B 



