56 EXPLANATION OF PLATES 33. 34. 



Plate 33. V. I. pp. 326, 327. Note. 

 Longitudinal Section of Nautilus Striatus, from the Lias 

 at Whitby, in the collection of Mrs. Murchison. The in- 

 terior of the Chambers is filled exclusively with calcareous 

 spar, and that of the Siphuncle with Lias. (Original.) 



a. The Siphuncle : the union of the siphuncular calca- 



reous sheaths, with the aperture or collar of each 

 transverse Plate, is so closely fitted, that no fluid 

 could have passed between them into the air cham- 

 bers. 



b. One of the transverse Plates forming the Air cham- 



bers. 



c. White calcareous spar, filling the middle region owZy 



of the air chambers. 



d. Stratified zones of dark coloured calcareous spar, de- 



posited in equal thickness on both sides of the 

 transverse plates, and also on the inside of the shell, 

 and around the calcareous sheath of the siphuncle.* 



e. Portion of the external shell, shewing a laminated 



structure. 



Plate 34. V. L p. 329. Note. 



Drawing of the animal of the Nautilus Pompilius, pre- 

 pared at my request by Mr. Owen, to shew the manner 

 in which the siphuncle terminates in the Pericardium. 

 (Original.) 



* The successive zones of this dark Spar shew that the Lime com- 

 posing it was introduced by slow and gradual infiltrations into the 

 cavity of the air chambers. Hence it follows that no communication 

 existed between the Siphuncle and these chambers, at the time when 

 this Pipe was filled with the fluid mud, that has formed a cast of Lias 

 within it. As the fractures across the Siphuncle in the 2nd and 3rd 

 chambers are filled only with spar, of the same kind as that within 

 these Chambers, these fractures could not have existed, when the 

 Mud of the Lias formation entered the Siphuncle, without admitting 

 it also into the chambers adjacent to them. 



