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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.

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