174 AXEL A. OLSSON 



of Santo Domingo and 0. tamiamiensis Mansfield of the Florida Miocene 

 which have a similar cellular formation to its shell. These oysters have 

 been referred to a separata genus Pycnodonte Fischer, the type species of 

 which is a Cretaceous species. 



Typical specimens have a round, thick shell with strong ribs, the in- 

 terior colored a porcelain-white with a wide purple border. Other specimens 

 which have grown attached to smooth piling will have a flat or curved 

 surface and a relatively thin shell without ribs. The outer layer is frequently 

 thin and of a lilac-pink or purple color and marked with fine, flowlike lines. 

 The colored marginal band will show on inspection the cellular structure 

 of the inner layer. 0. solida Sowerby, 1871 (in Reeve, Conch. Icon., pi. 14, 

 fig. 28. Gulf of Panama) may be the same species. 



Range — Lower California to Ecuador and the Galapagos Islands. 

 Ecuador: Atacames; Esmeraldas. 



SUBGENERIC POSITION UNCERTAIN 



Ostrea ( ) irideseens Gray Plate 22, figure 7; 



Plate 23, figures 2, 2a 



Ostrea irideseens Gray, 1854, in Hanley, Conch. Miscell., Ostrea, pi. 2, figs. 6, 7. — 



Hertlein and Strong, 1946, Zoologica, vol. 31, pt. 2, p. 55. 

 f Ostrea prismatica Gray, 1825, Ann. Phil., vol. 25. 



Shell often large, irregularly ovate to subrectangular, attached by 

 most of the surface of the lower valve, its outer edges only becoming free 

 and raised. The upper valve is flat or slightly vaulted. Inner layer of shell 

 is dense, porcellaneous and white, often showing an iridescent or strongly 

 metallic luster, often thickened with closely packed layers interfingered be- 

 tween prismatic and more chalky seams. The outer layer is relatively thin, 

 of a subhorny texture, concentrically fluted and radially wrinkled, and of 

 a bronze or purple color. This outer layer is easily destroyed, but when per- 

 fect extends as a wide margin or as a denticulated fringe beyond the white, 

 porcellaneous margin of the inner layer. Interior white with blotches of 

 purple or metallic brown. Dorsal lateral margins denticulate. 



This oyster grows to a large size and specimens 10 inches or more in 

 length are fairly common. The outer, subhorny, laminated layer is easily 

 lost by erosion, and is attacked by alkalies. Such worn valves are white, 

 often profusely veined with purple and could easily be mistaken for Ostrea 

 virginica as noted by Carpenter. This species grows attached to rocky 

 ledges exposed to heavy surf waves and in northern Peru, as well as else- 

 where, is much sought for as an article of seafood. 



Range — Lower California to northern Peru. Panama: Panama City, 

 Guanico. Ecuador: Esmeraldas; Manta. Peru: Caleto Mero; Caleto Sal; 

 Mancora. 



Ostrea ( ) tobulifera Dall Plate 17, figure 8 



Ostrea tubulifera Dall, 1914, Nautilus, vol. 28, No. 1, p. 3. 



Shell suborbicular, about 45 mm. in diameter, nearly flat, with a whitish 

 nacre, brilliantly poUshed inside, purple clouds showing through, and with 

 a translucent irregularly crenulated margin. The ligamentary area is nar- 



