16 MOLLUJSCA. 



is also upon conjectures of a similar nature that reposes the classifi- 

 cation of the 



AMMONITES, Brug. 



Or the Cornua-Ammoni, or horns of Ammon*, for they no longer 

 exist except among fossils. They are distinguished from the Nautili, 

 by their septa, which, instead of being plane or simply concave, are 

 angular and sometimes undulated, but most frequently slashed on the 

 edge like the leaf of an acanthus. The smallness of their last cell 

 seems to indicate that like the spirula they were internal shells. They 

 are very abundant in the strata of secondaiy mountains, where they 

 are found varying from the size of a lentil to that of a coach wheel. 

 Their subdivisions are based upon the variation of their volutes and 

 siphons. 



The name of Ammonites Lam., (Simplegades, Montf., 82) is parti- 

 cularly restricted to those species in which all the whorls are visible, 

 and their siphon near the marginf . 



They have lately been divided into the Ammonites planites, of 

 Haan, where the edge of the septa is foliaceous, and into the ceratites 

 of Haan, where it is simply angular and undulated. 



Those in which the last whorl envelopes all the others form the 

 Orbitutites, Lam., or the Globites, and Goniatites of Haan, or the Pela- 

 guses, Montf., 62. in all of which the siphon is situated as in the pre- 

 ceding ones. 



The Scaphites Sowerb., are those in which the whorls are conti- 

 guous and in the same plane, the last one excepted, which is detached 

 and reflexed on itself. ; 



Some, Baculites, Lam., are entirely straight without any spiral por- 

 tion whatever. 



Some of them are round , and others compressed. || The last some- 

 times have a lateral siphon. 



The first cells of some of them the Hamites Sowerb., are arcuated. 



Finally, those which vary most from the usual form of this family 

 are the Tv/rrilites^ Montf., 118, where the whorls, so far from running 



4to, 1827 ; and that of M. J. S. Miller on the same subject in the Geol. Trans., 

 second series, vol. II, part I, London, 1826. See also Sage, Journ. de Phys. an. 

 IX, and Raspail, Journ. des. Sc. d'Observ., second No. To this genus we refer the 

 Paclite Montf., 318; the Thalamule, 322; the Achtlo'ite, 358; the Cetocinc, 

 370 ; the Acame, 374; the Belemnite, 382 ; the Hibolite, 386 ; tlie Prorodrague, 

 390 ; the Pirgopole, 394, which are the cases of different species. As to the 

 Amimone, Id., 326 ; the Callirhoe, 362 ; the Chrisaorc, 378, they appear to be 

 mere nuclei or piles of alveoli detached from their cases. 



* So called from the resemblance of their volutes to those of a ram's horn. 



f The various species of Ammonites have long been collected and described, but 

 with less care than those of other shells. We may commence studying them in the 

 article Ammonite, Ency. Method. Vers. I, 28, and in that of M. de Roissy, in 

 Sonini's Buffon, Mollusca, V. 16. See also the Monograph of Haan, entitled 

 " Monographic Ammoniteorum et Goniateorum Specimen," Leid. 1325. 



I Sc. obliquus, Sowerb. ; Cuv., Oss. Foss., II, part II, pi. ii, f. 13. 



Baculites vertebrate, Montf. 342 ; Fauj., Mont, de St. Pierre, pi. xxi. 



|| The Tiranitc, Montf., 346; Walch., Petrif., Supp., pi. xii, constitutes the 

 genus RHABDITES of Haan, who refers the ICTHYOSARCOUTES of Desmar to it. 



