380 AFFINITIES OF CHAMBERED SHELLS. 
existing cognate genera of Cephalopods ; we 
cannot but infer that these extinct families filled 
a larger space, and performed more important 
functions among the inhabitants of the ancient 
seas, than are assigned to their few living repre- 
sentatives in our modern oceans. 
Concliislo7i, 
It results from the view we have taken of the 
zoological affinities between living and extinct 
species of chambered shells, that they are all 
connected by one plan of organization ; each 
forming a link in the common chain, which 
unites existing species with those that prevailed 
among the earliest conditions of life upon our 
globe; and all attesting the Identity of the design, 
that has effected so many similar ends through 
such a variety of instruments, the principle of 
whose construction is, in every species, funda- 
mentally the same. 
Throughout the various living and extinct 
genera of Chambered shells, the use of the air 
chambers and siphon, to adjust the specific 
gravity of the animals in rising and sinking, 
appears to have been identical. The addition 
of a new transverse plate within the conical 
shell added a new air chamber, larger than the 
preceding one, to counterbalance the increase of 
weight that attended the growth of the shell and 
body of all these animals. 
