334 THE WONDERS OF GEOLOGY. Lect. IV. 



27. The ammonite, or cornu ammonis. — The fossils 

 called Ammonites, first appear in the secondary formations ; 

 or more properly, no traces of their remains have been 

 found in the tertiary or any later deposits. The Ammo- 

 nite, so called from its supposed resemblance to the horn of 

 Jupiter Ammon, is a fossil chambered shell, coiled up 

 in the form of a disk, bearing a close analogy to the 

 nautilus, but differing in the situation of the siphunculus, 

 and in the septa by which the interior is divided. In the 

 Nautilus these partitions are entire, and their section pre- 

 sents a series of simple curves ; but in the Ammonite they 

 are very sinuous, and the external surface of the casts 

 commonly exhibits markings resembling the outlines of 

 deeply fringed foliage ; the shell is also generally decorated 

 with flutings, ribs, or tubercles. The siphuncle, or syphon, 



which in the Nautilus is 

 central, is situated at the 

 back, in the Ammonite. 

 I have placed before you 

 examples from the Gait 

 of Folkstone, in which 

 the shell remains, — from 

 Watchett and Hartwell, 

 with the internal na- 

 creous coat only, — while 



Lign. 65. — Ammonites communis; . . 



from the Lias of Whitby. in this common species 



from Whitby (Lign. 65), 



the shell is altogether wanting, the specimen being a cast 



of the interior, formed of argillaceous iron-stone ; a state 



in which these fossils are frequently found. 



In some examples, the shells and partitions of the cham- 

 bers having decomposed, casts of the cells have been formed, 

 which fit into each other, and admit of being put together, 

 so as to show the entire shape of the ammonite. Many 

 hundred kinds of Ammonites are known in the secondary 



