AMMONOCEEAS, OE AMMONOCEEATITES. 63 



whorls. This last character is used in De Blainville' System 

 to distinguish the Ammonacea from the Nautilacea. This 

 family contains the genera Discorbites, Scaphites, Ammonites, 

 and Simplegas. 



AMMONACEA. Lam. The seventh famUy of Polythalamous 

 Cephalopoda, Lam. containing the genera Ammonites, Orbulites, 

 Ammonoceras, Turrilites, and Baculites, to which may be added 

 Amalthus, Simplegas, Ellipsolites, JSTautellipsites, Hamites, 

 Icthyosarcolites, and other genera mentioned in the list of 

 figures 477 to 484. 



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

 Lam. and Bl. — Descr. Symmetrical, convolute, discoidal, orbi- 

 cular ; chambers numerous, divided by lobated, branched, or 

 siuuous 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 external structure is 

 exhibited by a section. There is some difficulty in distinguish- 

 ing them from the Eossil Nautili, for although the whorls, being 

 visible, and the septa sinuous, may be taken as the character- 

 istics of the Ammonites, yet there are several species which 

 partake the characters of both. The Orbulites of Lamarck 

 (fig. 479) for instance, have siuuous 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 ex- 

 ternally, and the septa simple. Ammonites is figured iu the 

 plates— PL xxii. fig. 478. 



AMMONOCEEAS, or ) Lam. (from Ammon & Kepac, ceras, 



AMMONOCEEATITES. 3 horn.) 



The shells described under this Lamarckian genus present an 

 anomaly which is considered by Mr. Gr. B. Sowerby, sen., as 

 merely accidental. They resemble the Ammonites in internal 

 structure, but instead of being spirally convolute, they are 

 merely curved like a horn. Ex. (copied from De Blamville) 

 PI. xxii. fig. 477. 



