87 
AMMONITES. 
Gen. Char. Volute, chambered, siphuncled, septa 
of the chambers undulated at their margins. 
AMMONITES discus. 
TAB. XII. 
Spec. Char. Discoid, outer edge acuminated; 
Aperture sagittate, half the diameter of the 
shell in length, and one sixth in breadth. Vo- 
lutions concealing each other. 
Shell apparently smooth on the outer surface, very much 
flattened and acuminated on the outer edge. Aperture 
spear-head shaped. Septa of the chambers irregularly un- 
dulated. About four inches in diameter, half an inch in 
thickness. 
This specimen is from the stone quarry near the house of 
industry at Bedford ; it is in the cabinet of the Rev- T. O. 
Marsh, who has favoured me with the loan of it. It is a 
rarity, and as far as I know, a new species. The umbili- 
cus is probably covered, which seldom happens with an 
Ammonites ; we see it on one side so nearly covered, al- 
though on the other it is less so, that we conclude it would 
be wholly covered in more perfect specimens. The septa 
show on one side their waved margins at different corroded 
depths in the shell, which is more than can be seen in more 
perfect specimens, and it is agreeable to learn by what is left 
us to distinguish species. A good Naturalist will know, 
sometimes, by a part the nature of the whole, in subjects of 
