54 EXPLANATION OF PLATE 32. 



a double keel-shaped indented process, enlarging from 

 its apex backwards.* (Original.) 



Plate 32. V. I. p. 244. 



Fig. 1. Part of the petrified shell, and casts of the interior 

 of some of the chambers, of a Nautilus hexagonus, 

 from Marcham, Berks. This fossil exhibits at its 

 smaller End, from d to b., a series of casts of the 

 Air chambers, from which the external shell has 

 been removed. The cavity of each chamber 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 forming minute Ribs 

 across its surface. (Original.) 



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

 reous 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 Calcare- 

 ous grit, similar to that which forms the rock from 

 which the shell was taken. See V. I. p. 247.f (Ori- 

 ginal.) 



* Although the resemblances between these fossil beaks, 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 Sepias that 

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

 us to distinguish between the Rhyncholites derived from naked Sepiae, 

 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. 



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



