13 / 
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 
chambers of the Cornu ammonis are distinguished by the name of 
Spondy lo lithe s . 
In the quarries of Wet and Dry Sandford, in Oxfordshire, are fre- 
quently found fossils of a very curious appearance, being the series ot 
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 actu- 
ally 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 Black- 
down. Some of these specimens are rendered particularly interesting 
by the conservation of the outer shell, now a white silicious substance, 
and apparently 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 mean- 
dering lines of the sutures, as they reach to the surface. 
T 
VOL. III. 
