64 EXPLANATION OF PLATE 32. 



a double keel-shaped indented process, enlarging 

 from its apex backwards.* (Original.) 



Plate 32. V. I. p. 323. 



Fig. 1. Pa t of the petrified shell, and casts of the 

 interior of some of the chambers, of a Nautilus 

 hexagonus, from Marcham, Berks. This fossil ex- 

 hibits at its smaller End, from d. to h.y a series of 

 casts of the Air chambers, from which the external 

 shell has been removed. The cavity of each cham- 

 ber is filled with a disc of pure calcareous spar, 

 representing the exact form of the chamber into 

 which it had been infiltrated. In the larger portion 

 of this fossil, the petrified shell retains its natural 

 place, and exhibits fine wavy lines of growth form- 

 ing minute Ribs across its surface. (Original.) 



Fig. 2. Fractured shell of N. hexagonus, from the Cal- 

 careous grit of Marcham. The chambers are lined 

 with calcareous spar, and a circular plate of 

 the same spar is crystallized around the siphon. 

 The interior of the siphon is filled with a cast of 

 Calcareous grit, similar to that which forms the 

 rock from which the shell was taken. See V. I. p. 

 326.t (Original.) 



* Although the resemblances between these fossil bealcs, and that 

 of the animal inhabiting the N. Pompilius, are such as to leave no 

 doubt that Rhyncholites are derived from some kind or other of 

 Cephalopod, yet, as they are found insulated in strata of Muschel 

 kalk and Lias, wherein there occur also the remains of Sepiae that 

 had no external shells, we have not yet sufficient evidence to enable 

 us to distinguish between the Rhyncholites derived from naked Sepias, 

 and those from Cephalopods that were connected with chambered 

 shells. I possess a specimen of a fossil Nautilus from the Lias at 

 Lyme Regis, in which the external open chamber contains a Rhyn- 

 cholite. 



t This fossil exhibits the Siphuncle in its proper place, passing 



