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THE LIAS AMMONITES. 
The 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 has disclosed the fact that the earlier Paleontologists established very many of 
their species on characters derived either from young specimens, or from fragments 
belonging to different phases of growth of the same species. This cause has helped to 
explain the hopeless nature of the task which the practical Palseontologist 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 many forms discloses 
the circumstance that the 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 many 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 this 
vast labyrinth of " species," and noted that one cause of the difficulty arose from the 
fragmentary nature of the materials at the disposal of the earlier observers, and likewise 
from the fact, unknown at the time, that most Ammonites change their form during 
growth, I felt it imperative on the Palaeontologist to examine the morphology of every 
species in order to place its history on a scientific basis. I have for many years pursued 
this method in the study of Liassic and Oolitic forms, and learned therefrom the numerous 
sources of error and perplexity which beset my early studies, and have discovered that 
many species pass through important changes of structure between youth, maturity, and 
old age, the amount of change varying considerably in the different generic groups, and 
that this character should form an important element in their diagnosis.^ 
After Leopold von Buch, the first great reform in the classification of the Ammonitid^ 
was made by my learned and esteenied friend Professor Edward Suess, of Vienna, who, in 
a memoir,^ ' Ueber Ammoniten,' communicated to the Vienna Academy, pointed out some 
important characters in the structure of the Ammonite shell which had been overlooked 
or underestimated by former observers, such as the form of the aperture and the 
structure of the central and lateral processes, which were often developed from its margin ; 
and, secondly, the size of the body-chamber, considered in relation to the length of 
the last whorl. The study of these characters of course necessitated the discovery and 
study of perfect specimens, and the whole anatomy of the shell received more attention from 
him than it had done in the hands of his predecessors. By an application of the facts 
obtained therefrom Suess established three new genera, Arcestes, Lytoceras, and FhijUo- 
ceras on a very solid foundation. Eive years later he extended his studies on the spiral 
1 "On the Development of Ammonites," 'British Association Reports,' Bath Meeting, p. 73, 1864. 
2 'Ueber Ammoniten,' erste Abth., Sitzungsbei". Nat.-wiss. Classe Wiener Akademie, Baud 52, 1S65. 
