370 UNITY OF DESIGN. 
varied, whilst the interior is similarly constructed 
in all of them, to act as a float, subservient to 
the movements of Cephalopodous MoUusks. We 
have seen that the Ammonites, beginning with 
the Transition strata, appear in all formations, 
until the termination of the Chalk, whilst the 
Hamites and Scaphites are very rare, and the 
Turrilites and Baculites do not appear at all, 
until the commencement of the Cretaceous for- 
mations. Having thus suddenly appeared, they 
became as suddenly extinct at the same period 
with the Ammonites, yielding up their place and 
office in the economy of nature to a lower order 
of Carnivorous moUusks in the Tertiary and ex- 
isting seas. 
In the review we have taken of genera in the 
family of Chambered shells, allied to Nautilus, 
and Ammonite, we have traced a connected series 
of delicate and nicely adjusted instruments, 
adapted to peculiar uses in the economy of 
every animal to which they were attached. 
These all attest undeviating Unity of design, 
pervading many varied adaptations of the same 
principle ; and afford cumulative evidence, not 
only of the exercise of Intelligence, but also of 
the same Intelligence through every period of 
time, in which these extinct races inhabited the 
ancient deep. 
