368 
were obviously free when they first entered them. If we admit 
such possibilities, and then find similar phenomena in the Paleozoic 
epoch, we shall no longer need our first picture, but can construct 
a far more natural one. 
The Nautiloidea will not then present themselves as a simple 
chain of being, but as they really were—several distinct stocks or 
grand series, arising from a common stock or radical, and each of 
these grand series divisible into many parallel lines of genetically 
connected forms. In the Lower Silurian, some of these do not 
have close-coiled forms at all; some of them have: but all, except 
the most primitive series, which are composed wholly of straight or 
arcuate forms, have some close-coiled species. These we can often 
trace directly with the greatest exactness, both by their develop- 
ment and by the gradations of the adult forms, to corresponding 
species among the straight shells. 
The series we have described above, from the straight Bactrites 
to Goniatites, compares closely with any single genetic series of the 
Nautiloidea, and shows that this last arose very suddenly in the 
Protozoic, and evolved true nautilian shells in the Calciferous and 
Quebec groups on the earliest fossiliferous level known positively to 
contain the remains of Cephalopoda. 
The genera of Ammonoidea evolved in the Silurian and Devonian 
are structurally much more distinct from each other than any groups 
of the same value (7. e., genera) in the succeeding formations, and 
thus, in different but equally plain characters, teach us that they 
also had.a quicker evolution within those periods than in the later 
formations. Either this was the case, or else the Ammonoidea were 
created in full possession of an organization only attained by 
similar parallel series of congeneric, close-coiled nautiloids, after 
passing through all the intermediate transformations above described. 
These comparisons bring out other curious results. Thus although 
both are orders and taxonomically equal, we cannot compare the 
whole of the Ammonoidea with the whole of the Nautiloidea, but 
only with a more or less perfect single series of that order. 
The radicals of the Nautiloidea, Diphragmoceras, Endoceras, 
Orthoceras and Cyrtoceras, evolve through time as an organic 
trunk giving off an indefinite number of small branches in Paleo- 
zoic time, each branch complete in itself and composed of suc- 
cessive species becoming more arcuate, coiled and closer coiled and 
finally involute. In the Trias the trunk comes to an end, but a small 
