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[1]according to the age of the strata in
- ↑ Thus one of the first forms under which this family appeared, the Ammonites Henslowi, (Pl. 40, Fig. 1,) ceased with the Transition formation; the A. Nodosus (Pl. 40, Figs. 4, 5.) began and terminated its period of existence with the Muschelkalk. Other genera and species of Ammonites, in like manner, begin and end with certain definite strata, in the Oolitic and Cretaceous formations; e. g. the A. Bucklandi (Pl. 37, Fig. 6.) is peculiar to the Lias; the A. Goodhalli to the Greensand; and the A. Rusticus to 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 to Geology, 1834, p. 77.
