450 



MOLLUSCA 



PHYLUM YI 



adductor ahsent ; edentulous, or luith ohscure schizodont dentition ; dimijarian when 

 young ; the foot obsolete or ahsent in the adidt. Carboniferous to Recent. 



Ostrea Linn. (Fig. 686). Shell irregulär, iiiequivalve, and witli terminal beaks, 



Fio. 686. 

 Ostrea digitalina Dubois. Miocene ; Vienna Basin. 



Fio. 687. 



Alectryonia (jregaria 

 (Sow.). Oxfordian ; 

 Dives, Calvados. 



witb radial or foliaceous sculpture, usually discrepant on the two valves. Some species 

 (0. virginica^ titan, gigantea, etc.) attain a very large size. Carboniferous to Recent. 



Alectryonia Fischer (Dendrostrea Swains ; Adinostreon Bayle) (Fig. 687). Left 

 valve attached to roots or branches by clasping shelly processes ; botli valves with 

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

 Jura and Cretaceous. 



Gryphaea Lam. {Pycnodonta Fisch.; Gryjjhaeostrea Conr.) (Figs. 688, 689). Left 



Fig. 688. 



Gryphaea arcuata Lam. 

 Lower Lias ; Pf obren, near 

 Donfiuescbingen, Baden. 



Fig. 689. 



Gryphaea vesicularis Lam. Wlxite Clialk 

 Isle of Rügen. 



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 ; Geratostreon, Aetostreon^ Ehynchostreon Bayle) 

 (Figä. 690, 691). Resembliug Gryphaea, but the valves more equal, hinge with an 

 obscure tooth, beaks of both valves niore or less spiral, the pit for the ligament narrow. 

 Upper Jura and Cretaceous. 



