﻿3C8 



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 (/. 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 



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