172 AXEL A. OLSSON 



Shell elongate, commonly arcuate, the beak of the lower valve being 

 usually curved strongly towards the posterior side. Lower valve is more 

 convex and heavy, the outside smooth or marked with weak riblets most 

 pronounced on the umbone, the general color white, tinged slightly with 

 violet. Upper valve is flat to depressed, smooth or concentrically lamellated 

 or wrinkled. Interior is white except for the adductor scars and sometimes 

 the edges of the cardinal area which are stained deep purple. Margins of 

 the shell white. Dorsal margins nondenticulate. 



As noted by Hertlein, this oyster has sometimes been confused with 

 0. chilensis Philippi but the Chilian species is a true Ostrea belonging to 

 the group of 0. edulis of Europe. 0. corteziensis resembles some forms of 

 O. virginica but is typically heavier and the riblets much weaker. 0. 

 corteziensis is common at Panama where it is gathered for food. 



Range — Gulf of California to Panama. Panama; Cathedral Rocks, 

 Panama City. For other records see Hertlein. 



Ostrea (Crassostrea) columblensis Hanley Plate 23, figures 4, 4a 



Ostrea columbiensis Hanrey, 1846, Proc. Zool. Soc. London, p. 187. — Sowerby, 1871, 

 Reeve, Conch. Icon., vol. 18, Ostrea, pi. 7, figs. 10a, b.— Dall, 1909, Proc. U. S. 

 Nat. Museum, vol. 37, pp. 149, 255, pi. 26, fig. 2.— Maxwell Smith, 1944, 

 Panamic Marine Shells, p. 51, fig. 677.— Hertlein and Strong, 1946, Zoologica, 

 vol. 31, pt. 2, p. 54. 



Shell relatively small (length to about 90 mm.), very irregular, the 

 attached valve deep, cup-shaped, the upper valve smaller and generally 

 flattened. Sculpture smooth or with crudely formed ribs. External color 

 usually a deep purple or a blue-black, the interior white except for a wide, 

 purple-colored marginal band. 



Generally attached to the roots of mangrove. Common along the 

 shore of the Gulf of Guayaquil; quantities of this shellfish are sold in the 

 markets of Guayaquil. 



Range — Lower California to northern Peru. Peru: Lower Tumbez. 

 Ecuador: Mangrove shores of Rio de Oro. 



Subgenus ALECTRYOJfIA Fischer de Waldheim, 1810 



Type species by subsequent designation, Stoliczka, 187L Ostrea crista^ 

 galli Linne. Indo-Pacific. (Often known as Lopha Bolten, 1798); for a 

 discussion of the rejection of the name "Lopha" see Stenzel, 1947, Jour. 

 Pal., vol. 21, No. 2, pp. 169, 177. 



Shell large or small, rounded, subovate to elongate, falcate or strongly 

 sickle-shaped, the two valves of similar shape and sculpture. Sculpture 

 consists of large, rounded ribs or sharp plications which deeply affect the 

 margins. Margins of the valve near the beak and cardinal area are crenulated 

 or finely pustulated. 



Ostrea (Alectryonia) megodon Hanley Plate 23, figures 3, 3a 



Ostrea megodon Hanley, 1846, Proc. Zool. Soc. London, p. 106. — Sowerby in Reeve, 

 1871, Conch. Icon., vol. 18, Ostrea. pi. 12, figs. 24a, 24b.— Maxwell Smith, 1944, 

 Panamic Marine Shells, p. 51, fig. 692K.— Hertlein and Strong, 1946, Zoologica, 

 vol. 31, pt. 2, p. 55.— Durham, 1950, Mem. Geol. Soc. America, No. 43, pt. 2, 

 p. 59, pi. 5, fig. 3. 



