215 
AMMONITES armatus. 
TAB. XCV. 
Spec. Char, Involute, volutions exposed, 'with 
many annular undulations armed with two rows 
of large short furrowed spines. 
Volutions six or seven, pressed against each other, the 
undulations continue upon the spines, and after meeting on 
the point pass on to the other side: there are about five 
undulations upon each spine, and one between each. The 
aperture is obscurely four-sided, the inner side being 
smallest and concave. When the shell is broken off the 
cast the spines are succeeded by an ovate flat disk, over 
which the margins of the septa are beautifully delineated. 
The siphuncle is visible, as in most of the Genus, near the 
front. The young shell is plain and without the spines. 
Numerous varieties of this species are found in the great 
Alum-clay formation at Whitby, where this large-sized speci- 
men was gathered by Mr. Strangewayes, by whose favor I have 
thus been enabled to draw a larger specimen than I possessed. 
We have here also the advantage of many specimens, the 
middle being a small plain one of which many are occasion- 
ally found, which indeed might have been considered a dif- 
ferent species ; the next circle might by the same rule form 
a second species with larger radii, and again the third with 
the flat disks and fewer striae than the outer circles. I have 
separate specimens of most of these different sizes ; they are 
liable to be destitute of the inner volutions. It is worthy of 
