157 



Fig. 7, from Folkstone. The internal arrangement of these septa 

 will be understood from Fig. 9, where a section of the same fossil is 

 shown. 



From this sinuous disposition of the septa, the chambers necessarily 

 derive a very peculiar form : a form hardly to be described ; but of 

 which a correct idea may be obtained from the figure Plate IX. Fig. 3, 

 which is a representation of one of the casts which are sometimes 

 formed in the chambers of these shells ; the surrounding shell having 

 been removed by decomposition, subsequent to having been filled with 

 the matter of which these casts are formed. These casts of the cham- 

 bers of the Cornu ammonis are distinguished by the name of Spon- 

 dylolithcs. 



In the quarries of Wet and Dry Sandford, in Oxfordshire, are fre- 

 quently found fossils of a very curious appearance, being the series of 

 casts, in the chambers of this shell, formed of spathose matter, the shell 

 itself having been totally removed. In these fossils, which bear the 

 general form of the shell, the casts of the chambers, though actually 

 distinct, are so closely locked into each other, as frequently to render 

 their separation very difficult. They are known among the quarrymen 

 by the name of jointed snake stones. 



I have two specimens from Wiltshire, in which the Cornu ammonis has 

 been imbedded in flint, and in which the terminations of the septa are 

 still to be seen, now formed of flint. 



The terminations of the septa are very beautifully preserved, in snow- 

 white filaments, in some of the silicious specimens from Blackdown. 

 Some of these specimens are rendered particularly interesting by the con- 

 servation of the outer shell, now a white silicious substance, and appa- 

 rently the thickness of the original shell. A species of Ammonites, found 

 at Yeovil, which is filled with a very fine white lime-stone and spar, 

 yields a very beautiful appearance,, when the outer surface is so far 

 rubbed down, as to show the elegantly meandering lines of the sutures, 

 as they reach to the surface, 



VOL. III. T 



