OHDKR I 



PRIONODESMACEA 



373 



o.s-/ />.?, Linn. (Fig. 637). Shell invgular, iiie<|iiival \v, ami with ti-nninal beaks, 

 with radial or foliaceous sculpture, usually discrepant <>n the two valves. Sum.- 



FIG. 637. 

 Ostrea <li</UaUna, Dubois. Miocene; Vienna Basin. 



species (0. virginica, 0. titan, 0. gigantea, etc.) 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 



Fio. 638. 



Alectr i/nniii ijri ijn riii, Sow. 

 sp. Oxfoniian ; Dives, 

 Calvados. 



<!i-in>li<ii'it uri-i'ntii, Lam. 

 Lower Lias; 1't'oliren, near 

 Donaneaohingen, Baden. 



tiniiilin'ii 

 Isle of Riigen. 



White Chalk; 



strong, often divaricate folds and undulate margins. Trias to Recent ; maximum in 

 Jura and Cretaceous. 



Gryphaea, Lam. (Pycnodonta, Fisch. ; Gryphaeostrea, Conr.), (Figs. 639, 640). L-t'i 

 valve strongly arched, with iucurvi-d Leak, sessile when young, later free; right valve 

 flat and opercular. Lias to Tertiary ; chiefly Mesoxoic. 



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. 



