58 EXPLANATION OF PLATES 35. 36. 



Plate 35. V. I. p. 339. 



Cast of the interior of the Shell of Ammonites obtusus 

 from Lyme. Fragments of the shell remain near b. and e. 



One object of this Plate and of many of the figures at 

 PI. 37. is to shew the manner in which the external shell is 

 fortified by Ribs and Flutings, (PP. 340. 341.) and further 

 supported by the edges of the internal transverse plates, 

 that form the air chambers. See V. L p. 348, Note. 

 (Original.) 



Plate 36. V. L p. 338. Note. 



Longitudinal section of another shell of Ammonites ob- 

 tusus from the Lias at Lyme Regis. (Original.) 



The greater part of the outer chamber, and the entire 

 cavities of the air chambers are filled with calcareous spar, 

 and the Siphuncle, (preserved in a carbonaceous state,) is 

 seen passing along the entire dorsal margin to the com- 

 mencement of the outer chamber. See V. L p. 351, Note. 



Von Buch has found evidence to shew that the mem- 

 branous siphuncle of Ammonites was continued to a con- 

 siderable distance along the outer chamber, beyond the last 

 or largest transverse Plate. This discovery accords with 

 the analogies afforded by the membranous neck of the 

 siphon of the N. Pompilius, which is continued along the 

 outer chamber from the last transverse Plate to the Peri- 

 cardium. See PI. 34. q.^ 



Gizzard, the lateral pressure of these two organs on the neck of the 

 Siphuncle would tend to close it with a force exactly counterbalancing 

 the external pressure on the Pericardium. 



* As the body of the animals that inhabited the Ammonites was 

 more elongated than that of those inhabiting the shells of Nautili, 

 in consequence of the smaller Diameter of their outer Chamber, the 

 place of their Heart was probably more distant from the last trans- 

 verse Plate, than that of the Heart of Nautili; and the membranous 

 Siphon connected with the Pericardium consequently longer. 



