OP CONCHOLOGY. 31 



A very convex shell, which, when perfect, seems to have been 

 unusually long on the posterior side, and more convex than any 

 living species with which I am acquainted. It is. remarkable for 

 its excessive thinness, which is so marked that the specimen 

 before me shows distinctly the color of the internal coat, through 

 the shell substance. 



MYTILUS.Linn. 



M. UNGULATUS, Linn. 



Mytilus ungulatus, Linn., GmeL, Syst. Nat. No. 12. 



A common species of the Chilian Coast. In a loose sandstone, 

 probably the same as that containing the first two species of this 

 paper. 



ARCA, Linn. 

 S. Gren. ScAPiiARCA, Gray. 

 A. (S.) Raimondii, Gabb. 



Shell long, slender, sub-compressed, very inequilateral ; an- 

 terior end rounded ; posterior end very obliquely rounded, sub- 

 truncate ; base nearly straight ; beaks about a third of the length 

 from the anterior end, approximate, incurved and pointed 

 slightly in advance. Area narrow, about two-thirds as long as 

 the shell. Surface marked by about thirty square radiating 

 ribs with flat interspaces of about equal width. Internal mar- 

 gin marked by folds corresponding to the external ribs. 



Length 2 in., width 1*2 in., diameter '75 in. 



Not unlike A. lima, Rve., Icon. Conch., No. 101, but less 

 rounded in general outline. The base is more straight, the pos- 

 terior truncation more marked and the beaks more prominent. 



AXIN^A, Poll. 

 A. Paytensis, D'Orb., sp. 



Pectuncidus Paytensis, D'Orb., Amer. Merid. p. 129, pi. 15, 



fig. 11—13. 

 Pectunculus Paytensis, D'Orb., Prod. Pal. Strat. Vol. 3, p. 



122, No. 2307. 



One of the most common shells at the locality, judging from 

 the number of specimens I have seen. Prof. Orton collected 

 numerous specimens, and Dr. Raimondi sent me two fine ones. 



