62 AMMONOCERATITES. 



AMIMONUS. Montf. Conilites ungulatus, Knorr. A species 

 distinguished only by being slightly curved ; Fam. Orthocerata, 

 Lam. 



AMMONACEA. Bl. The fourth family of the order Polythalamia, 

 Bl. or chambered shells, described as thin, chambered, discoidal, 

 convolute, symmetrical, generally compressed, with visible whorls. 

 This last character is used in De Blainville's System to dis- 

 tinguish the Ammonacea from the Nautilacea. This family 

 contains the genera Discorbites, Scaphites, Ammonites, and 

 Simplegas. 



AMMONACEA. Lam. The seventh family of Polythalamous 

 Cephalopoda, Lam. containing the genera Ammonites, Orbulites, 

 Ammonoceras, Turrilites and Baculites, to which may be added 

 Amalthus, Simplegas, Ellipsolites, Nautellipsites, Hamites, Icthy- 

 osavcolites, and other genera mentioned in the list of figures 

 4/7 to 484. 



AMMONITES. Auct. (from Jupiter Ammon.) Fam. Ammonacea, 

 Lam. and Bl. — Descr. Symmetrical, convolute, discoidal, orbicular; 

 chambers numerous, divided by lobated, branched or sinuous 

 septa, perforated by a Siphon ; aperture generally more or less 

 modified by the last whorl. The fossils of the secondary strata 

 which compose this genus are numerous and well known ; they 

 are vulgarly termed ". snake-stones," and some of them are 

 extremely beautiful, particularly when the internal structure is 

 exhibited by a section. There is some difficulty in distinguishing 

 them from the Fossil Nautili, for although the whorls, being visible 

 and the Septa sinuous, may be taken as the characteristics of the 

 Ammonites, yet there are several species which partake the 

 characters of both. The Orbulites of Lamarck (fig. 479) for 

 instance, have sinuous septa like Ammonites, but the last whorl 

 covers those which precede it as in Nautilus- Simplegas 

 Montf. and Bl (fig. 475) has the whorls visible externally and 

 the septa simple. Ammonites is figured in the plates (478). 



AMMONOCERAS, orj ■' ' : 



AMMONOCERATITES. \ W (fr ° m Jmmon & K ^ cera8 ' hom " ) 

 The shells described under this Lamarckian genus present an 



