MADE BY DR. ANTONIO RAIMONDI IN PERU. 297 



RemarJcs. A fine large species, at once recognizable by its outline and its few, 

 large, symmetrically arranged ribs. It was figured without a name in the report 

 of the Wilkes' Exploring Expedition. 



O. LARVIFOEMIS, n. s., PI. 42, fig. 3. 



Shell small, very inequilateral, oblique, arcuate, nearly equivalve ; beaks ter- 

 minal ; posterior side and base forming together a semicircle ; anterior side pro- 

 duced, somewhat excavated above ; surface marked by about seven large radiating 

 ribs, alternating on the opposite valves ; the posterior two or three dividing into 

 two near the margin. All crossed by rough lines of growth. Interior unknown. 



Figure, Twice natural size. 



Locality. " Hill of three crosses, between Aguamiro and Huallacan, Prov. of 

 Huamalies; height more than 4000 metres." Considered Cretaceous by Dr. 

 Raimondi. 



Remarks. A pretty little oyster of the type of O. larva, but much shorter, 

 broader, and heavier than that species. From a notch near the beak, it seems to 

 have grown attached to a twig, in the manner of the recent mangrove oysters. 



EXOGTRA, Say. 

 E. POLYGONA, Yon Buch, Petr. rec. en Amer., p. 5, PL 2, f. 18-19. 



Two fine specimens of this strongly characterized species from between Com- 

 bayo and PoUoc ; Dept. of Cajamarca. Dr. Raimondi says " calcareous rocks 

 which seem to belong to the Jurassic formation." D'Orbigny refers the species to 

 the Neocomien. Should the small Exogyra found with Ammonites Raimofidianus 

 prove to be this species, it may have to be placed higher in the Cretaceous. See 

 remarks on E. plicata. 



E., sp. indet., PI. 8, fig. 4, 4a. 



A small shell from the same locality as the preceding, showing the exsert spiral 

 beak characteristic of the genus, but too imperfect to describe. It seems to be 

 somewhat of the type of E. arietina, E.oem., of the Texas Cretaceous, but not so 

 developed in the beak. The surface is very convex, especially in the middle, and 

 seems to have been smooth. Its entire length is barely three-fourths of an inch. 



E. PLICATA, Lam. 



For synonymy see Palaeontology of California, vol. 2, p. 2t5. 



Two specimens from the same locality as the other two species, and which 

 seem to belong to this well-known form. Their surfaces are somewhat obscured 

 by the matrix, and one at least is marked by unusually small ribs. This, however, 

 is not a valid specific distinction in this genus. 



