Fig. 2. Cast of part of a straight-chambered shell (Baculites Fraujasii, of Lamarck), in which the septa, or partitions, are deeply and regularly sinuated. In fossils of this kind, the cast of each chamber is distinct from the others; but the series is held together by the flexuosities of the septa. From Maestricht.
Fig. 3. A limestone cast of the chamber of an Ammonite: from Bath. The elongated channel in the middle indicates the position of the siphuncle.
Fig. 4. Fragment of an Ammonite, showing cavities of two chambers, and the canal of the siphuncle, partly lined with calcareous spar.
Fig. 5. Polished sections of an Ammonite (Ammonites Walcotii) from the Lias, Whitby. The chambers are filled with semi-transparent spar. The siphunculus is seen running along the dorsal, or outer margins of the volutions. The dark appearances observable in several parts of the siphuncle result from the carbonization of the animal membrane with which the tube was lined in the living state.
Fig. 6. "An Oval Ammonite."—Mr. Parkinson. This is evidently the cast of a discoidal shell pressed into an elliptical form. In the Chalk-marl, casts of Ammonites, Nautilites, &c. are very commonly more or less distorted by compression. The marl appears to have remained in a plastic state after the decomposition of the shell in which it was moulded, and to have admitted of being squeezed into close contact with the surrounding matrix; when the stratum became consolidated the cast retained its accidental shape, and adhering but slightly to the investing marl, was separable by a properly directed blow. This explains the otherwise unintelligible fact of a cast being closely invested by the rock, and all traces of the shell in which it was formed absent. When both the cast and the matrix became solid and uncompressible before the shell was decomposed, then loose casts were formed; as is common in the Portland stone, &c. The fossil figured appears to be an indifferent example of a common chalk-marl species (Ammonites Mantelli, of Sowerby).
Fig. 7. A beautiful cast of an Ammonite, in which the foliaceous septa transmuted into pyrites (sulphuret of iron, or marcasite), are exquisitely shown.
Fig. 8. A very fine specimen of an Ammonite (Ammonites latus, of Sowerby), from the "Galt;" a subdivision of the Lower chalk, in which Ammonites, with their pearly shells beautifully preserved, are abundant. From Folkstone, in Kent; a celebrated locality for these and other fossils of the same cretaceous deposits.
Fig. 9. Sections of a pyritous cast of an Ammonite, showing the sinuous edges of the septa.