Eve 
earlier observers, and likewise from the fact, unknown to them 
at the time, that most Ammonites change their form during 
growth, so that it became imperative on the Palzontologist 
to examine the morphology of every species, in order to place 
its history on a scientific basis. Having 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. I have discovered 
likewise that many species pass through important morphological 
changes of structure between youth, maturity, and old age, and 
that the amount of change varies considerably in the different 
generic groups; so that this character forms an important 
element in their diagnosis. . 
After Lzorotp von Bucn, the first great reform in the 
classification of the AmmoniT1p# was made by my learned and 
esteemed friend Professor Epwarp Svusss, of Vienna, who, in 
a memoir,* “ Ueber Ammoniten,” communicated to the Vienna 
Academy, pointed out some important characters in the struc- 
ture 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 dwelling 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, 
and by an application of the facts obtained therefrom Svuzss 
established three new genera, Arcestes, Lytoceras, and Phylloceras 
on a very solid foundation. Five years later he extended his 
studies on the spiral of the Ammonite shell,+ “‘ Die zusammen- 
setzung der Spiralen Schale.” The reform so well inaugurated 
* “Ueber Ammoniten,” erste Abth., Sitzungsber. der Nat.-wiss. Classe 
der Wiener Akademie, 1865, Band 52. Abth. 1. 
+ “Ueber Ammoniten,” 2 Abth., Sitzungsber, der Mathemat. Classe der 
Mathemat. Classe der Wiener Akademie, 1870, Band 61, Abth. 1. 
02 
