AMMONITES excavatus. 
TAB. CV. 
Spec. Cuar. Involute, lenticular, subumbilicate; 
keel sharpish, crenulated; whorls about six, 
those of the young shell exposed ; inner margin 
nearly right-angled; radii curved, obscure in 
full grown shells; aperture in adult shells 
sagittate; inner angles truncated. 
(+ SS ee 
A surcur concavity separates the keel from the remaining 
uniformly convex sides; the radii, which in the last-formed 
whorls of full-grown shells are little more than irregular 
strie, are in the first whorls very prominent over the inner 
angles; these angles in the young shell are rounded, and 
have not the flat space which in the old shells forms a 
partial umbilicus. The length of the aperture is about half 
that of the diameter of the shell, and the width at the back 
about one-third. The chambers are rather distinct. 
I had the pleasure of picking this up a few years ago on 
the productive hill of Shotover near Oxford, and as I saw 
no more specimens it may possibly be rare. The shell 
which is replaced by Carbonate of Lime is moderately 
thick, and appears very exact in most parts. The cham- 
bers are commonly hollow, and crystallized within; the 
thin septa are also distinctly replaced by Carbonate of Lime. 
The finishing chamber, which would add a fourth to the 
size of the whole, and which I have separate, is filled with 
