LOWER CRETACEOUS FORMATIONS AND FAUNAS 607 
Cretaceous is at the top of the Comanche series, and this plane 
seems to coincide very closely with the top of the Potomac and 
the top of the Shasta. 
It has already been mentioned that the Cretaceous of part of 
the west coast of South America has some resemblance to the 
Comanche series, but little is published about the stratigraphy, 
and in describing the fossils few attempts have been made to 
indicate the different horizons, so that it is now impossible to 
make close comparisons. In Peru Schloenbachia acuto-carinata, 
and several large Naticas and Tylostomas, occur with a few 
other forms related to Comanche species. Similarly in Colum- 
bia Ptychomya buchiana (Karsten), Exogyra boussingaulti d’Orb. 
and several others have representatives in the Comanche. It 
has been assumed by some authors that these forms lived in the 
Pacific basin, and that they prove that the Pacific and Atlantic 
were connected during Lower Cretaceous time. 
By independent comparisons with European Cretaceous 
faunas— principally northern European in one case and southern 
or Mediterranean in the other—aided by stratigraphic relation 
with overlying beds and by the evidence of the fossil plants we 
have reached the conclusion that the Shasta group and the 
Comanche series were essentially contemporaneous deposits. 
And yet their faunas are almost totally distinct. This separate- 
ness of the two faunas has frequently been mentioned by Dr. 
White? and the present writer,” but the real character of the dif- 
ferences has not been clearly Stated.« simemuiherences are, not 
dependent on fine discrimination of closely related species. On 
the contrary, whole genera and in some cases much higher 
groups that are abundant and characteristic in one area are 
entirely absent in the other. The following table will exhibit 
some of the more striking contrasts : 
tBull. U. S. Geol. Surv., No. 15, 1885, pp. 30-31; zdzd., No. 82, 1891, pp. 
180-108. 
2Bull. Geol. Soc. Am., Vol. 1V, 1893, p. 254; JouR. GEOL, Vol. III, 1895, pp. 
858-861; Bull. U. S. Geol. Sury., No. 133, 1896, pp. 27, 31; DILLER and STANTON; 
Bull. Geo. Soc. Am., Vol. V, 1894, p. 462. 
