62 EXPLANATION OF PLATE 42. 



Fig. 2. Cast of a chamber of Ammonites excavatus, 

 having a complex form derived from the denticulated 

 edges of the transverse plates. See V. I. pp. 264, 

 265, Note. (Original.) 



Fig. 3. Casts of three chambers of Ammonites catena, 

 with the Membrane of the Siphuncle on its dorsal 

 margin. See V. I. p. 264, Note, and p. 265, Note. 



The course of the transverse plates is beneath 

 the depressed and weakest parts of the external shell, 

 avoiding the bosses at c, d, e, which from their form 

 are strong. (Original.) 



Fig. 4. Ammonites varicosus, from the Green Sand of 

 Earl Stoke, Wilts. Nat. size. See V. I. p. 265, Note. 

 (Original.) 



Figs. 5. 6. Portions of the same shell, having the trans- 

 verse Plates and Siphuncle converted to Calcedony. 

 See V. I. pp. 265 and 266, Note. (Original.) 



Fig. 7. Ammonites varicostatus, (nobis,) an undescribed 

 species of Ammonite from the Oxford Clay at 

 Hawnes, 4 m. S. of Bedford. Diameter 9 inches. 



The name Varicostatus expresses the remarkable 

 change in the character of the Ribs, near the outer 

 termination of the air chambers. 



On the inner whorls of the shell, these ribs are 

 narrow, and highly raised, set close to one another, 

 and bifurcated at the back of the shell, (from d. to 

 c.) ; but near the outer chamber (b. to a.) they be- 

 come broad and distant, and the dorsal bifurcation 

 ceases. 



The edges of the transverse plates are exposed by 

 the removal of the shell from c. to b., they appear 

 also at a. d. (Original.) 



Similar variations in the form of the ribs occur in 

 Ammonites biplicatus and Ammonites decipiens. 



