INHABITING MULTILOCULAR SHELLS. 49 
universal occurrence of these Ammonites, of which not a single liy- 
ing analogue is found to tell the history of a genus once, perhaps, 
the most abundantly diffused of any ever created. 
The variety of form in these fossils is, of course, very great, and 
there are whole groups, or sub-genera, characteristic of certain forma- 
tions, and never found in any other. Thus, the Goniatites form the 
first sub-genus, and occur only in the mountain limestone and older 
beds. The Ceratites are peculiar to the muschelkalk, a peculiar 
continental stratum, occurring between the upper and lower beds of 
the new red sandstone of English Geology. The oolite formations 
are provided with quite a series of sub-genera, confined to them in 
local distribution ; and the cretaceous system, though not character- 
ized by its own group of these fossils, does not want for species 
found only in that formation. It is necessary that we should first 
explain the grounds of distinction in these sub-genera, and then, 
because of their superior importance, we shall enlarge a little on the 
Goniatites and Ceratites, two groups departing more than any other 
from the ordinary type of the genus. 
The shell of the Ammonite is, as we have said, of a simple flat, 
spiral shape, and is formed of a succession of chambers separated by 
thin plates of carbonate of lime, called septa, which, however, are 
perforated, and allow a tube to pass through all of them to the first 
formed chamber. ‘These perforations, too, are always on the back 
of the shell. Now, it is quite clear that a succession of these trans- 
verse plates, in a shell like that of the Ammonite, cannot but 
strengthen the shell very much, and enable it to resist a pressure and 
an amount of external injury which would otherwise crush and de- 
stroy it. It is also true, though not, perhaps, quite so clear, that if 
these plates, instead of being flat, are irregularly puckered or bent in 
and out, by a series of folds all ending in the centre of the plate, 
and making the line of intersection of the septum of the shell a com- 
plicated curve, instead of a straight line, the strength will be very 
much increased by an increase in the weight of material. This is a 
truth sufficiently well known to practical men, and acted on in the 
construction of cast-iron columns, which are always stroager for the 
same weight when they are fluted. The septa of the chambers of 
Ammonites are thus fluted, and often in a most complicated way, so 
as to present a very beautiful and remarkable appearance, somewhat 
resembling the edge of a parsley leaf, when the line of intersection 
with the shell is open to our view. In the Nautilus, and generally in 
all the species referred to the family Nautilacea, there is none of this 
VOL. 1X., NO. XXV. 7 
