58 
EXPLANATION OP 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. I. p. 348, Note. 
(Original.) 
Plate 36. V. I. 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 Siph uncle, (preserved in a carbonaceous state,) is 
seen passing along the entire dorsal margin to the com- 
mencement of the outer chamber. See V. 1. 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 affbi'ded 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 of 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. 
