PANAMIC-PACIFIC PELECYPODA 171 



Superfamily OSTKACEA 



Family OSTKELDAE 



Shell porcellaneous, nionomyarian or with one adductor muscle scar 

 in each valve, attached by cementation by its umbone or whole outer sur- 

 face of the left valve to rocky ledges, sometimes to roots of trees, and the 

 like. Valves generally much distorted through fixation, the lower or left 

 valve generally larger and deeper, the upper valve often flat. Sculpture 

 alike or markedly different on the two valves. Marine and brackish. 



Genus OSTREA Liim6, 1758 



Type species by subsequent designation. Children, 1823, Ostrea edulis 

 Linne; also Gray, 1847, same species. Recent, seas of Europe. 



Ostrea is a composite genus of great antiquity; fossil species are re- 

 corded from rocks as old as the early Mesozoic. Recent species can be 

 distributed amongst several well-marked groups on basis of shape, structure 

 of shell, and surface sculpture but authorities differ widely as to the syste- 

 matic importance which should be accorded these groups. For the sake of 

 simplicity, the groups herein recognized, will be considered as subgenera 

 of Ostrea. 



Key to species of Ostrea 



A. Upper and lower valves unlike in sculpture and form. 



I. Lateral margins near the beak without crenulations. 



Subgenus Crassostrea 



II. Lateral margins near the beak crenulated. 



1. Attached valve subovate, deeply concave, with ribbed external sculp- 

 ture, the upper or free valve flat, depressed, smooth, or with concentric 

 sculpture only. 



Subgenus Ostrea, s.s. 



B. Valves with similar sculpture but often differing greatly in size and 

 shape. 



III. Shell with strong ribs or plications in both valves. 



Subgenus Alectryonia 



IV. No ribbed sculpture, the surface of the upper valve with concentric 

 markings primarily. 



Group of 0. iridescens 

 Subgenus OSTREA s.s. 

 This group is apparently not represented in Panama-Pacific waters. 

 Ostrea chilensis Philippi, the common edible oyster of Chile belongs here. 



Subgenus CRASSOSTREA Sacco, 1887 

 Type species by original designation, Ostrea virginica Gmelin. Recent, 

 east American. 



Shell irregular in form, generally more or less elongated. Attached 

 valve is the larger and deeper, usually marked externally by a series of 

 small riblets which may become raised or subspinose; the upper valve of 

 the same general form but flatter and smoother. Adductor scars often 

 colored purple. Lateral margins internally smooth. 



Ostrea (Crassostrea) cortezlensis Hertlein Plate 23. figures 1, la 



Ostrea corteziensis Hertlein, 1951, Bull. South. California Acad. Sciences, vol. 50, pt. 

 2, pp. 68, 69, pi. 24, figs. 1, 2; pi. 26, fig. 7. 



