AMMONITES. 333 
SECTION IV. 
AMMONITES. 
Having entered thus largely into the history of 
the Mechanism of the shells of Nautili, we have 
hereby prepared ourselves for the consideration 
of that of the kindred family of Ammonites, in 
which all the essential parts are so similar in 
principle to those of the shells of Nautili, as to 
leave no doubt that they were subservient to a 
like purpose in the economy of the numerous 
extinct species of Cephalopodous Mollusks, from 
which these Ammonites have been derived. 
Geological Distribution of Ammonites. 
The family of Ammonites extends through the 
entire series of the fossiliferous Formations, from 
the Transition strata to the Chalk inclusive. 
M. Brochant, in his Translation of De la Beche's 
Manual of Geology, enumerates 270 species ; 
these species differ according to the age of the 
strata in which they are found, f and vary in 
t Thus one of the first forms under which this family appeared, 
the Ammonites Henslowi, (PI. 40, Fig. 1), ceased with the 
Transition formation ; the A. Nodosus (PI. 40, Figs. 4, 5.) 
began and terminated its period of existence with the Muschel- 
Kalk. Other genera and species of Ammonites, in like manner, 
begin and end with certain definite strata, in the Oolitic and Cre- 
taceous formations; e. g. the A. Bucklandi (PI. 37, Fig. 6.) is 
