162 M. Duval on the Belemnites of the Lower Cretaceous 
he has thus been led; but to shew the most prominent fea- 
tures of his work, it will be sufficient to indicate some of the 
facts which this observer has established. 
The naturalists who have treated of Belemnites are not 
agreed as to the degree of importance which ought to be at- 
tached to differences of form in these fossils; and in order to 
shew to what an extent this difference of opinion has been 
carried, it may be sufficient to state, that thirty-three of the 
species described by M. Raspail are referred by M. d’Or- 
bigny to one and the same species—namely, the Belemnites 
dilatatus of M. de Blainville. This is owing to the first of 
these authors regarding all the variations of external form as 
characteristic of distinct species, while M. d’Orbigny looks 
upon these variations as being dependent for the most part 
on the changes which the age of the animal occasions in the 
shape of the shell. This latter opinion was supported by 
powerful arguments, but its accuracy was not demonstrated ; 
and we were not in possession of a certain rule for distinguish- 
ing the specific peculiarities of different individuals, produced 
by the progress of its growth. Now, this rule has been defi- 
nitely laid down by M. Duval; and, in the greater part of in- 
stances it admits of no uncertainty. 
Belemnites are found to be composed of two principal parts 
—namely, a conical alveolus or socket, a kind of cup divided 
by partitions, open in front, and a sort of sheath covering 
this socket, and prolonged more or less posteriorly, so as to 
form a rostrum directed backwards. The cup or socket in- 
creases by the formation of new chambers placed before those 
already existing, and secreted by an organ lodged in the inte- 
rior; the rostrum, on the contrary, acquires size nearly in the 
same manner as the stalk of an exogenous plant, by the suc- 
cessive deposit of layers applied exteriorly to the more ancient 
layers, and produced very probably by the action of a part 
which, in its turn, covered all this portion of the shell. ‘These 
superimposed layers are, in general, very distinct among them- 
selves; and, consequently, by making suitable sections of the 
Belemnite, it becomes easy to ascertain in an adult individual 
the form which it must have borne after the deposition of 
each of these plates or layers—that is to say, at different pe- 
