280 OBSERVATIONS ON THE ANIMALS 
vantage of entering at a sharp angle is considerable—remembering 
also the flattened shape in the latter family, and the amount in 
which it differed from Nautilus in its other proportions, and at the 
same time not losing sight of the numerous analogous points—it 
will probably be admitted that the anatomical and physiological 
structure of the types of the two principal families of Cephalopods 
must have been, on the whole, a good deal alike, differing, however, 
probably, in some not unimportant points; and then comes the 
question of more general interest, and more fit, indeed, for discus- 
sion—How did this change show itself in the habits of the animal ? 
It must not be supposed that I have been making these extended 
remarks with a view of preparing the reader for some grand theory 
to which I am about to advert; my object has rather been to 
give an account of the methods of reasoning by which naturalists 
have come to the conclusion that Ammonites, Hamites, &c., be- 
longed to animals, in all probability, nearly allied to, and yet dis- 
tinct from, those inhabiting the Nautilus and other shells, bearing 
considerable resemblance in outward form. It is well that these 
foundations of opinions should be generally understood ; for it is by 
no means the case that a mere superficial resemblance is at all suffi- 
cient tu prove that a real affinity exists. 
Now with regard to the change of habits. The animal of the 
Nautilus, we know, floats occasionally at the surface of the water ; 
but from the remains of food which have been found in the stomach, 
and also from its occurring generally near sandy shores, there seems 
a great probability that it feeds on the smail crustacea inhabiting 
the bottom, at no great depth of water. It will be clear that the 
larger the size of the siphuncle the more rapidly could the specific 
gravity of the mass be changed, and an upward or downward mo- 
tion communicated, and thus that the Nautilus is peculiarly fitted 
for shallow sandy and muddy bottoms, and the neighbourhood of 
small islands. It seems, however, equally evident that the animal 
would never be very likely to burrow in the mud or be enveloped 
in it, endowed as it was with powers of locomotion to a considera- 
ble extent. 
Now, during the deposition of those extensive beds which com- 
pose the oolitic formation, and which we have already mentioned as 
peculiarly rich in Ammonites, the bottom of the sea must have con- 
sisted chiefly of calcareous mud ; and there is good reason to sup- 
pose that while the deposition was going on the depth of water 
could not have been very great in the places where the Ammonites 
are found, for along with them there occur numerous organic re- 
