OSTREACEA. 



695 



shell-structure is laminated. This family is exclusively confined to 

 salt water, and comprises the Oysters. Numerous Secondary and 

 Tertiary representatives of the family are known, but the few Palae- 

 ozoic types which have been referred here are probably really of a 

 different nature. 



The principal genus included in the Ostreidce, as here understood, 

 is Ostrea itself, the essential characters of which are those of the 



Fig. 557. — Ostrea (Exogyra) CoulonL Lower Greensand. 



family itself. The shell described by De Koninck from the Car- 

 boniferous Limestone under the name of Ostrea nobilissima has 

 generally been quoted as the oldest known type of the genus Ostrea, 

 but it is referred by Fischer to the genus Pachypteria (? Spondylitics). 

 In the Secondary and Tertiary rocks, however, we meet with a vast 



Fig. 558. — Ostrea (Alectryonia) Marshii. 

 Oxford Clay (Middle Oolites). 



Fig. 559. — Gryphcza incurva. Lias. 



number of fossil Oysters, which have commonly been distributed 

 among several more or less well-marked sections of the genus, of 

 which the following are the most important. In the typical forms 

 of the genus {Ostrea proper) the valves are pretty nearly of the same 

 length, the lip of the right valve is not serrated, and the external 



