TOSSIL FAUNA. 139 



PLATE LX. 



Ammonites. 



Fig. 1. A Belemnlte {Belemnitella mucronatd) attached to a flint. Kent. 



Fig. 2. Cast of part of a straight-chambered shell {Baculites Fraujasii, of Lamarck), in which the 

 septa, or partitions, are deeply and regularlj' 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 wliich 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 Port- 

 land stone, 8sc. The fossil figured appears to be an indifiTerent example of a common 

 chalk-marl species {Ammonites MantelU, 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 {Ammo7iites Imittis, of Sowerby), from the " Gait;" 

 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. 



