24 
AMMONITES. Hippocastanum. 
TAB. DXIV.—fig. 2: 
Spec. Cuar. Gibbose, umbilicated, radiated, spi- 
nose; inner whorls almost concealed ; radii 
ten or more, unequal, much elevated, each fur- 
nished with three tubercles upon the front, and 
most of them with two obtuse spines upon each 
side ; aperture transversely obovate. 
Disrixeuisuen from A. rhotomagensis by the convex- 
ity of the sides of the whorls, the small number and thick- 
ness of the radii, and the size of the spiniform tubercles ; 
the thickness equals two-thirds of the diameter at least. 
At first sight this fossil has a very striking appearance, 
although upon examination it is found to be very nearly 
allied to the following. 
The specimen is in the cabinet of H. 'T. De la Beche, 
Ksq., who discovered it at Dowlands, in the same Chalk 
(replete with large grains of Quartz and Green-sand,) 
which produced the Pachymya (tab. 504.). We havea 
small specimen from near Rouén, also containing parti- 
cles of Green-sand, but no conspicuous grains of Quartz. 
We have named it from its resemblance to the thorny 
capsule of the Horse-chestnut. 
