the others are intermediate ; fig. 3 shewing long radii 

 with large tubercles ; and figures 4 and 5 irregular radii 

 with undefined tubercles. 



AMMONITES tuberculatus. 

 TAB. CCCX,— Figs, 1, 2, and 3. 

 Spec. Char. Depressed, radiated ; volutions gib- 

 bose, half concealed, front rather flat ; radii 

 arising- in threes from large round tubercles 

 and uniting in pairs to form large compressed 

 tubercles upon each edge of the front ; aper- 

 ture suborbicular. 



JL HE sides of the whorls are very convex especially in 

 voung shells which have the aperture transverse, the 

 round tubercles are placed near the middle of the side of 

 the whorl ; the radii after uniting to form the flat tuber- 

 cles upon each edge of the fronts, bend forward and 

 continue over it as far as the siphuncle, which they meet 

 at nn acute angle ; the length of the mouth is about two- 

 iifths the diameter of the last whorl, its width variable 

 with age. 



AMMONITES proboscideiis. 

 TAB. CCCX.— Figs. 4 and 6. 



Spec. Char. Depressed, tuberculated ; volutions ven- 

 tricose, partly concealed ; front concave ; tubercles 

 upon the sides of the last whorl and both edges of 

 the front, subcylindrical ; aperture orbicular, 

 var. a, tubercles connected by very irregular and short 

 radii. — var. /3, destitute of radii. 



A HE orbicular aperture, and elongated nearly cylindrical 

 tubercles, distinguish this species : there are about eight sets 

 of tubercles only upon each whorl, in the preceding species 

 there are 13 or more : the front is broad but not well defined, 

 and th-e siphuncle very large. The aperture is about one- 

 third the diameter of the shell wide. 



The four species of Ammonites above described are all from 

 a stratum of Marie below the Chalk at Folkstone ; they are also 

 found in a similar stratum at Cambridge and other places. It 

 is difficult to divide the species, as there appears to be a regular 

 series from that without tubercles (A. dentatus) to one witli 

 large flat tubercles upon the margin (A. auritus, M. C. t. 134-,) 

 and to another with cylindrical tubercles at each end of the 

 rays, (A. proboscideus) ; the extremes are, however, so dis- 

 tinct, that it is difficult to refuse them the rank of species. 

 They are all splendidly pearly shells, but are very apt to be 

 destroyed by the decomposition of the Pyrites, with which they 

 are commonly more or less completely filled; those parts that 

 are not pyritous are in the Folkstone specimens filled with klack 

 indurated Marie. 



