130 Studies in Geology, No. 3 



smaller forms only exist. Careful comparison of growth- 

 stages in the large and small specimens has led to the con- 

 clusion that they are of the same species. The younger 

 forms are more produced posteriorly, and thus lower than 

 the larger ones, which are subequilateral, but tracing out of 

 the growth-lines on larger forms corresponding to the 

 margins of the younger ones reveals a clear identity of 

 shape. 



C. nelsoni is very much like several forms from the Carib- 

 bean and Floridian Miocene. C. reevei Gabb,^^" from Gatun. 

 is different in being broader posteriorly, having a broader 

 surface behind the posterior keel, with (apparently) not so 

 prominent a sulcus behind the keel, and in its sculpture, 

 which is concentrically waved over the entire surface, where- 

 as nelsoni has the waved sculpture on the beaks only. C. 

 (Scamhida) denstis Dall,^^^ from the Oak Grove beds of 

 Florida, is very close, probably the most closely related form 

 outside of Peru. The only point of difference in the mature 

 shells of the two species, disregarding size, is a greater pos- 

 terior attenuation in nelsoni; densus is much more evenly 

 elliptical in cross-section viewed dorsally. The mature shells 

 of nelsoni are somewhat larger. 



C. (Scainbnla) nielinus var. meridionalis DalP^^ is also 

 very close, but is higher, less inflated, and the ventral margin 

 does not ascend posteriorly as does that of nelsoni. C. 

 (Scamhiila) deformis Heilprin "° is somewhat similar, but 

 differs in having pronounced undulatory sculpture, and in 

 the resilium, which is narrower than in nelsoni^ and oval 

 instead of triangular in shape. 



"'Brown and Pilsbry, Proc. Ac. Nat. Sci. Phila., vol. 64, p. 515. 

 pi. 24, fig. 5. 1912. 



"* Trans. Wag. Inst., vol. 3, pt. 6, p. 1472, pi. 39, figs. 9, 10, 11, 12, 

 1903. 



""Idem., p. 1473, pi. 37, figs. 6, 13. 



""Dall Bull. U. S. Nat. Mus. 90, p. 131, pi. 22, figs. 6, 7, I9i5- 



