INHABITING MULTILOCULAR SHELLS. 37 
however, a vitality in nature’s arrangements, and a principle of com- 
pensation, which quite precludes any danger of this kind. Dr. Buck- 
land suggests that the carnivorous Trachelipods might have supplied 
their place, and quotes the observations and opinion of Mr. Dillwyn 
to show the probability of this substitution. Mr. Dillwyn observes 
that all the herbivorous marine Trachelipods of the older strata were 
furnished with an operculum, as if to protect them against the carni- 
yorous Cephalopods which then prevailed abundantly ; but that in the 
tertiary formations numerous herbivorous genera appear which are 
not furnished with opercula, as if no longer requiring the protection 
of such a shield after the extinction of Ammonites and other power- 
ful enemies of that class. Dr. Buckland remarks upon this that we 
may see good reason to adopt the conclusion of Mr. Dillwyn, “ that 
in the formations above the chalk the vast and sudden increase of one 
predaceous tribe has been provided for by the creation of many new 
genera and species possessed of similar appetencies, and yet formed 
for obtaining their prey by habits entirely different from those of the 
Cephalopods.” 
We may add that during the tertiary period, at least the earlier 
part of it, one genus of these, the Cerilhium, existed in most extraor- 
dinary abundance and variety, probably clearing away the exuberance 
of life which must at that time have characterized the country now 
forming the richest and most thickly peopled districts of France. 
However, be the explanation as it may, the fact is undeniable.-— 
The Cephalopods have diminished in abundance. Instead of being 
the most common, they may now be ranked amongst uncommon ani- 
mals; andthe perfect stability of nature in this really most important 
change is a remarkable instance of the power of adaptation in the eco~ 
nomy of the world to every variety of circumstances and every alteration 
of the conditions of life. No sooner is the pressure, as it were, taken 
off by the gradual or rapid removal of a class of predaceous animals, 
than another starts up to take its place. No sooner does a species 
become scarce than want of food also reduces the number of its ene- 
mies; and thus the great laws which govern the world act harmoni- 
ously, and in dependence on each other, never deviating far from the 
strict and definite line drawn by the unseen hand of the Great Guider 
and Director of these laws. 
Proceeding now to the question of more importance—that of clas- 
sification, we must first direct attention to the difference in structure 
which separates all the real chambered shells into two great orders, 
named by M. D’Orbigny the Siphonifera and Foraminifera. There 
