OSTREID.E. 507 



Ex. P. sella, Gmelin, pi. 129, fig. 4, 4, a. 

 The curious striated and bent-up "Saddle Oyster" (P. sella) 

 is the best known of the species of this genus, which are 

 but few in number, and inhabit the sandy shores of China, 

 India, and Northern Australia. 



Species of Plaeuna. 



Lincolnii, Gray. sella, Gmel. 



papyracea, Lam. 



Fain. OSTREIDiE. 



Labial appendages triangular, connected around the mouth 

 by a plain membrane ; palps separate from the gills. Mantle 

 entirely open, the edges double, and each bordered by short 

 tentacular fringes ; no conspicuous ocelli ; branchial leaf- 

 lets not doubled on themselves. Foot obsolete. 



Shell irregular, foliated, free, or adherent. Ligament in- 

 ternal, or semi-internal. Muscular impression sub-central; 

 pallial line entire. 



Genus OSTREA, Linnaeus. 



Shell variously shaped, inequivalve, more or less inequi- 

 lateral, often thick and irregular; upper valve most fre- 

 quently flat or concave, under valve convex ; surface foliated, 

 or with radiating furrows. Hinge without teeth ; ligament 

 lodged in a pit in each valve, the beak behind that of the 

 under valve, often produced, and exhibiting a furrowed liga- 

 mental area. Muscular scar sub-central ; pallial impression 

 obscure, entire. 



