ORDER I PRIONODESMACEA 369 
ternal; shell structure coarsely prismatic, with a thin, partial, nacreous lining ; byssifer- 
ous. Devonian to Recent. 
Palaeopinna, Hall. Devonian ; North America. 
Aviculopinna, Meek. A very small wing in front of the beaks. Carboniferous 
and Permian. 
Pinna, Linn. (Fig. 620).. Shell thin, with a long hinge line; valves carinate, 
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, Morch. Jura to Recent. 
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, the wing divided internally by a longitudinal ridge ; 
dimyarian, the anterior adductor scars smaller ; equivalve 
more or less gaping behind; Schizodont, with a single 
antervor lateral, and an obscure or obsolete cardinal tubercle ; 
area ill-defined, amphidetic; ligament external, parivincular ; 
shell structwre cancellate, or built up of hollow prisms resem- 
bling those of Pinna, but not solid ; valves thick, internally 
marginate ; byssiferous (?); marine. Silurian to Carbon- Fic. 622. 
iferous. Conocardium alaeforme, Sow. Car- 
boniferous Limestone: Tournay, 
This group includes Conocardiwm, Bronn; (Plewro- Belgium. 1). 
rhynchus, Phill.), (Fig. 622); and Rhipidocardium, 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 adductor 
absent in the adult ; inequivalve, teeth irregular or absent, with a serial multivincular 
ligament ; byssiferous, with a moderate gape, or none. Permian to Recent. 

Fia. 623. 
A, Gervillia aviculoides, Sow. Oxfordian ; Dives, Calvados. B, (7. linearis, Buvignier. 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, alate behind, three to four den- 
ticulations under the beaks. Permian. 
VOL. I 2B 
