AMMONITES. 



Ammonites and Orbulites, Lam. Anim. sans vert. 

 Ammonoceras, ejusd. 



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TESTA discoidea, convoluta, polythalamia, an- 

 fractibus eontiguis, marginibus septorum loba- 

 tis et siniiosis, siphone dorsali. 



Among the various fossil shells which abound in the 

 secondary beds, and which are not known in a recent 

 state, one of the most remarkable and numerous is the 

 Genus Ammonites, commonly called Cornu Ammonis from 

 its resemblance to the convoluted horn generally repre- 

 sented on the head of Jupiter Ammon in Mythological 

 History. Snake-stone is also a name that is topically 

 applied to these fossils, and their resemblance to a coiled 

 snake is surreptitiously aided by an artificially formed 

 head. This Geuus, which consists of discoid, convoluted, 

 chambered shells with contiguous volutions, the margins 

 of whose septa are lobated and sinuous, and whose siphun- 

 culus is dorsal, is very nearly related to Nautilus, differing 

 from it principally in the position of the siphunculus, and 

 in the circumstance of the lobated and sinuous edges of 

 the Septa. We unite Lamarck's Orbulites and his Ammo- 

 noceras with /Jmmonites, because we cannot discover any 

 sufficient distinguishing character. The circumstance in 

 which they differ, and which has caused Lamarck to 

 separate them, is, that in the Orbulites the last volutions 

 covers all the former, while in Amrnonites all the volutions 

 are apparent; this, however, is not a sufficient character, 

 because there are connecting species, and if it were 

 admitted, it would be necessary to separate the only three 

 known recent species of Nautilus into two genera: the 

 existence of an umbilicus can never be regarded as a 

 generic distinction, inasmuch as in some instances it is 



