Maryland Geological Survey 551 



Pteria petrosa Conrad, probably its closest relative, has a strongly 

 arcuate basal margin, is very much less oblique and is smoothly rounded 

 instead of being inflated merely along the diagonal. 



Occurrence. — Monmouth Formation. Brightseat, and 1 mile west of 

 Friendly, Prince George's County. 



Collection. — Maryland Geological Survey. 



Superfamily OSTRACEA 



Family OSTREIDAE 

 Genus OSTREA Lamarck 

 [Prodrome, 1799, p. 81] 



Type. — Ostrea edulis Linne. 



Ostrea, the common oyster, is, doubtless, by reason of its great economic 

 value, the most widely known of any of the molluscan genera. The shell 

 is inequivalve, usually irregular and more or less inequilateral. Except- 

 ing in the larval stages it is attached by the convex left valve. The right 

 valve, which is flattened or slightly concave, serves as a cover. The hinge 

 is edentulous. There is a single muscle scar, the posterior, and this is 

 subcentral. The pallial line is simple, but not well defined. 



The genus has been prominent in all the molluscan faunas from the 

 Mesozoic onward, and more than two hundred and forty species have 

 been recognized in the Cretaceous alone. 



A. Radial sculpture developed in the right valve. 



1. Shell more or less falcate, often auriculate; radial sculpture not 



confined to the extreme margin of the shell nor 

 evanescing with conspicuous abruptness. 



a. External surface undulated or plicated Ostrea larva s. 1. 



i. Radial plications produced into umbonal region. 



Ostrea larva var. falcata 

 ii. Radial sculpture not produced into umbonal region. 



a'. Shell relatively large, auriculate, radials undula- 



tory, not sharply plicate. . . .Ostrea larva var. nasuta 

 b'. Shell relatively small, often auriculate, sharply 



plicate Ostrea larva var. mesenterica 



b. External surface linearly sulcate or finely corrugated. 



Ostrea plumosa 



2. Radial sculpture confined to the extreme margin, evanescing away 



from it with conspicuous abruptness. 



a. Outline ovate or elliptical, rarely arcuate. Ventral margin 



crenulated Ostrea monmouthensis 



b. Outline arcuate-elliptical ; ventral margin simple ; crenulations 



confined to convex margin Ostrea faba 



Etymology. — Ostrea, the Latin word meaning " oyster." 



