98 The American Geologist. February. i90i. 
The cephalopods of the Cambro-Silurian were small. Or- 
thoceras (Kionoceras) laqueatiim, O. sordium, O. cornu-oryx^ 
O. hilineatiim, 0. explorator of the Chazy or Calciferous, and 
the species determined by Dwight, those of Billings' Palaeo- 
zoic fossils and Hall's examples were nearly all narrow, short 
species. In the Trenton the large Orthocerata appear. Sim- 
ilarly the Lower Silurian Cyrtoceras, Gomphoceras, Phrag- 
moceras precede the Niagara forms sensibly heavier and more 
developed. 
The trilobites which were large in the Cambrian, and thin 
skinned ( ?), become in later ages more calcified, heavier cara- 
paced, with enlarged pygidia. 
Variant, as at some points this conclusion may seem, the 
conclusion of growth in size and increased functions of mineral 
secretion is almost invariably forced upon us from an inspec- 
tion of the fossil lines of growth. It utterly dispenses with 
any claim of novelty. But it seems as if it might be more 
generally recognized as a guide in the correlation of the hori- 
zons, geographically separated and characterized by the same 
fossil species. If we find a faunal expression in forms strongly 
developed, with larger fossils, and associated with increased 
skeletal deposition of parts, it is a reasonable conclusion that, 
apart from any or all considerations of improved food supply^ 
climatic auspices, or freedom from enemies, the horizon is sub- 
sequent to those in other sections, possibly holding the same 
species, in which these fossils are less developed, generally 
smaller and unassociated with morphological complexity. 
Questions of correlation are deservedly regarded as involv- 
ing a momentous risk, in so far as dogmatic assertion goes,, 
and the hope entertained to find in biological relations a defi- 
nite guide has been realized only within indeterminate limits. 
Huxley's well accepted idea of "homotaxis" shows the un- 
reasonableness of any exact inference as to simultaneity of 
faunas where there are the same fossil species, and writers on 
correlation have recognized the precarious virtue of an appeal 
solely to identity or similarity of specific forms. 
Russell (Correlation Papers; The Newark System) has 
said, "the usefulness of the life history of the earth as a means 
of correlating geological terranes is still further complicated 
by a principle inherent in life itself; that is, development has 
