169 
AMMONITES rotundas. 
TAB. CCXCIII .—Fig. 3. 
Spec. Char. Discoid; volutions exposed, costated; 
costa; thick, numerous, split over the front ; 
aperture orbicular. 
P rom the small piece I possess of this I am not able 
to make a very full description, but there is enough to 
distinguish it from the last; the costae are less numerous, 
much thicker, shorter and not so regularly divided into 
two over the front, in a short fragment some are not 
divided others are split into three; the sides of the 
whorls are not compressed, on the contrary they are 
rather ventricose, so that the aperture is orbicular, ex- 
cept a small sinus produced by the preceding whorl. 
From Purbeck, probably found in the Kimmeridge 
Clay. 
AMMONITES decipiens. 
TAB.— CCXCIY. 
Spec Char. Discoid, depressed ; volutions ex- 
posed, costated ; costae large, obscure towards 
the front; front covered by numerous small 
ribs ; aperture oblong. 
F our or five volutions compose small specimens of this 
shell, they are compressed on the sides and round in 
front; the numerous large costae are much elevated to- 
