450 



MOLLUSCA 



PHYLUM VI 



adductor ahsent ; edentulous, or with obscure schizodont dentition; dimyarian when 

 young ; the foot obsolete or absent in the adult. Carboniferous to Eeceiit. 



Ostrea Linn. (Fig. 686). Shell irregular, inequivalve, and witli terminal beaks, 



Fiii. CSO. 

 Ostrea digitalina Dubois. Miocene ; Vienna Bnsin. 



Fig. 687. 



Alectryonia grrgaria 

 (Sow.). Oxfordian ; 

 Dives, Calvados. 



with radial or foliaceous sculpture, usually disci-epant on tlie two valves. Some species 

 (0. virfiinica, titan, giyantea, etc.) attain a very large size. Carboniferous to Kecent. 



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

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

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

 Jura and Cretaceous. 



Gryfhaea Lam. {Pycnodonta Fisch. ; Gryphaeostrea Conr.) (Figs. 688, 689). Left 



Pio. GSS. 



Gryphaea areuata Lam. 

 Lower Lias ; Pfoliren, near 

 Donaueseliinfren, Baden. 



Pio. 089. 



Grypluwa ivsicvlarls Lam. White ClialU ; 

 Isle of Rugen. 



valve strongly arched, ^\■ith incurved l)calv, sessile wlien young, later free ; right valve 

 fiat and opercular. Lias to Tertiary ; cliiefly Mesozoic. 



Exogyra Say [Am^thidonta Fischer ; Geratostreon, Aetostrem, Bhynchostreon Bayle) 

 (Figs. 690, 691). Eesembliug Gry^ihaea, Ijut the valves more equal, hinge Avith an 

 obscure tooth, beaks of both valves more or less spiral, the pit for the ligament uarrow. 

 Upper Juj'a and Cretaceous. 



