Lign. 22:—Ammonites from the cretaceous formation.
| 1. | Ammonites varians, from Hamsey. |
| 2. | A. Dufresnoyi: 2a, part of the same. |
| 3. | A. lautus: 3a, keel and septum of the same. |
The Ammonites differ from the Nautili in having the margins of the septa or internal shelly partitions (which in the latter are smooth), foliated or wrinkled; and the siphunculus or tube placed along the back of the shell, whereas in the Nautilus it is central. The sides of the shell in the Ammonites are very generally more or less ornamented with arched elevations and depressions, and studded with spines and tubercles, as in the specimens above figured.
There are several kinds of Ammonites found in the Lias at Whitby and other places in Yorkshire; the most common species is figured in [Lign. 7. p. 20]; the dark colour of this fossil is produced by the argillaceous stone with which it is now filled. The internal structure of these Ammonites is generally well preserved, the chambers being lined with spar or other mineral matter; transverse polished sections are often very beautiful from the several cells being occupied by variously coloured marble, susceptible of a high polish. ([Pl. III.]) In some examples the entire shell is transmuted into brilliant pyrites (sulphuret of iron), and the chambers are filled with white spar; a specimen of this kind in my possession, collected by Lady Murchison, is the most elegant fossil imaginable.
Plate III.
Polished section of an Ammonite.