ORDER I PRIONODESMACEA 373 
Ostrea, Linn. (Fig. 637). Shell irregular, inequivalve, and with terminal beaks, 
with radial or foliaceous sculpture, usually discrepant on the two valves. Some 








if 
Fia. 637. 
Ostrea digitalina, Dubois. Miocene; Vienna Basin. 
species (0. virginica, O. titan, O. gigantea, ete.) attain a very large size. Carboniferous 
to Recent. 
Alectryonia, Fischer (Dendrostrea, Swains. ; Actinostreon, Bayle), (Fig. 638). Left 
valve attached to roots or branches by clasping shelly processes; both valves with 

Fic. 638. Fic. 639. 
Alectryonia gregaria, Sow. Gryphaea  arcuata, Lam. Fic. 640, 
sp. Oxfordian; Dives, Lower Lias; Pfohren, near Gryphaea vesieularis, Lam. White Chalk ; 
Calvados. Donaueschingen, Baden. Isle of Riigen. 
strong, often divaricate folds and undulate margins. Trias to Recent; maximum in 
Jura and Cretaceous. 
Gryphaea, Lam. (Pycnodonta, Fisch. ; Gryphaeostrea, Conr.), (Figs. 639, 640). Left 
valve strongly arched, with incurved beak, sessile when young, later free; right valve 
flat and opercular. Lias to Tertiary ; chiefly Mesozoic. 
Exogyra, Say (Amphidonta, Fischer ; Ceratostreon, Aetostreon, Rhynchostreon, Bayle), 
(Figs. 641, 642). Resembling Gryphaea, but the valves more equal, hinge with an 
obscure tooth, beaks of both valves more or less spiral, the pit for the ligament narrow. 
Upper Jura and Cretaceous. 
