[061]

Fig. l. Cast of a single chamber of Nautilus hexagonus, shewing the simple curvatures of the edges of the transverse plates, and the place of the Siphuncle. (Original.) [062]

Fig. 2. Cast of a chamber of Ammomtes excavatus,
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 Siphuncle 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.)

Fig. 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 transverse Plates and Siphuncle converted to Chalcedony. 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 Variocostatus 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.