252 AMMONITES. 



SECTION IV. 



AMMONITES. 



Having entered thus largely into the history of the Me- 

 chanism 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 Amjnonites. 



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 Manuel of Geology, enumerates 270 species ; 

 these species differ* according to the age of the strata in 



* Thus one of the first forms under which this family appeared, the Am- 

 monites Henslowi, (PI. 40, Fig. 1,) ceased with the Transition formation ; 

 the A. Nodosus (PI. 40, Figs. 4, 5.) began and terminated its period of e.t. 

 istence with the Muschclkalk. Other genera and species of Ammonites, in 

 like manner, begin and end witli certain definite strata, in tlic Oolitic and 

 Cretaceous formations ; e. g, tiie A. Bucklandi (PI. 37, Fig. 6.) is peculiar 

 to the Lias; the A. Goodhalli to the Greensand; and the A. Rustieusto the 

 Chalk. There are few, if any, species which extend through the whole of 

 the Secondary periods, or which have passed into the Secondary, from the 

 Transition period. 



The following Tabular Arrangement of the distribution of Ammonites, in 

 different geological formations, is given by Professor Phillips in his Guide 

 ■1o Geology, 1834, p. 77. 



