62 EXPLANATION OF PLATE 42. 
Fig. 2. Cast of a chamber of Ammonites excavatu.s, 
having a complex form derived from the denticulated 
edges of the transverse plates. See V. I. pp. 350, 
351, Note. (Original.) 
Fig. 3. Casts of three chambers of Ammonites catena, 
with the Membrane of the Siplmncle on its dorsal 
margin. See V. I. p. 350, Note, and p. 351, 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.) 
Fie. 4. Ammonites varicosus, from the Green Sand of 
Earl Stoke, Wilts. Nat. size. See V. I. p. 351, 
Note. (Original.) 
Figs. 5. 6. Portions of the same shell, having the trans- 
verse Plates and Siphuncle converted to Chalce- 
dony. See V. I. pp. 351 and 352, Note. (Original.) 
Fig. 7. Ammonites variocostatus, (nobis,) an undescribed 
species of Ammonite from the Oxford Clay at 
Hawnes, 4 m. S. of Bedford. Diameter 9 inches. 
The name V ariocostatus expresses the remark- 
able 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 h., they appear 
also at a. d. (Original.) 
Similar variations in the form of the ribs occur 
in Ammonites biplicatus and Ammonites decipiens. 
