669 
‘he describes the transformations they undergo, and investigates the 
laws of such changes. The chambers, or the internal characteristics 
of Ammonites, the importance of which was long ago indicated by 
Von Buch, have presented new featuresto M. d’Orbigny, which are 
easily applied to the purposes of classification. I speak of the di- 
stinction of the “ selles et lobes en parties paires et impaires,” accord- 
ing as they are cloven at the extremity, or terminate in a conical 
point. Combining this characteristic with that of the length of the 
dorsal lobe, and with those afforded by the exterior ornaments of 
the shell, the form of the back and mouth, between which there is 
almost always a coincidence, M. d’Orbigny has made twenty-one 
natural groups, of which eleven had been already established by 
Von Buch, and ten are new. Of these twenty-one groups, seven are 
peculiar to the Jurassic or Oolitic formations, ten to the cretaceous, 
and four contain species common to both. 
M. d’Orbigny points out the modifications of species through 
time and space, and shows the relation that exists between certain 
forms and the beds which contain them. He recognizes three 
new creations or replacements of the species of Ammonites during 
the cretaceous period, and thus establishes, on zoological data, three 
divisions of natural groups;—first the Neocomian*, second, the 
Gault, and third, the Upper Greensand (Craie chloritée), and the 
White Chalk; and he estimates that in this triple succession of de- 
posits, the Ammonites gradually decrease according to the numbers 
seventy-five, forty-two, and twenty-seven, to disappear finally with 
the uppermost chalk or Maestricht beds, and before the tertiary 
epoch. 
The total number of determined species of Ammonites in the great 
eretaceous system of France is 144, according to M. d’Orbigny, and 
with the exception of three, which are common to the Gault and 
the Upper Greensand, all the other species are divisible into groups, 
each of which is peculiar to one of the three great divisions of this 
system, and may be considered characteristic of it. Although the 
species have been thus replaced several times during the cretaceous 
period, there exists, however, among them a certain affinity of forms 
which differs sufficiently from the general characteristics of the Juras- 
sic Ammonites to constitute the beds containing them a truly distinct 
and separate series. We may congratulate M. d’Orbigny on having. 
begun his ‘ Paleontology ’ with the fossils of this period: for whilst 
the labours of the English, particularly the admirable general views 
and detailed descriptions of Dr. Fitton, and the works of Dr. Mantell, 
have contributed to a good acquaintance with the northern chalk and 
greensand, it must be confessed that there is ample room for research 
in the southern type. 
* We have to learn why the very well-defined British formation, the Lower 
Greensand, seems to be suppressed and merged by our opposite neighbours 
in the “ Systeme Néocomien.”’ Cannot the Lower Greensand be pre- 
served and the Neocomian be considered as a marine equivalent of our 
Wealden? 
