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The form of the spiral described by the growth of the shell 
has been taken as the basis for the establishment of several 
genera by Parxinson, Lamarcxk, D’Orpigny, Haver, and 
Liveitiz, and from this character alone we have the following 
sixteen genera. 
Ancyloceras, D’'ORB. Hamulina, D’ORB. 
Anisoceras, D’ORB. Helicoceras, D’ORB. 
Baculina, D’ORB. Heteroceras, D’ORB. 
Baculites, LAM. Ptychoceras, D’ORB. 
Choristoceras, Hav. Rhabdoceras, Hav. 
Cochloceras, Hav. Scaphites, PARK. 
Crioceras, Linv. Toxoceras, D’ORB. 
Hamites, PARK. Turrilites, LAM. 
This character, derived from the spiral of the shell, is only 
valuable when taken in connection with others, to which it 
becomes subordinate. _ 
The careful study of the immense collections of Ammonites 
that have been slowly accumulating during the last twenty-five 
years from the Triassic, Jurassic, and Cretaceous rocks, have 
disclosed the fact that the earlier Paleontologists established 
many of their species on characters which were often derived 
from young specimens, or from fragments belonging to different 
phases of growth of the same species. This fact has helped to 
explain the hopeless nature of the task which the practical 
Paleontologist had to encounter some years ago, when he 
attempted to correlate species found in strata of the same age 
in different regions of Europe, so that an analysis of the 
synonyms of several forms disclosed the fact that many species 
had been described under different names by the same author, 
from his having unwittingly studied different stages of growth 
of the same shell, an error which was only detected several years 
afterwards, when more perfect specimens of the fossil had been 
discovered, so that much confusion resulted from premature 
attempts to generalise when the necessary details were absent. 
Having often lost my way in these specific labyrinths, and 
discovered that one cause of the difficulty arose from the 
fragmentary nature of the materials at the disposal of the 
