THOCHO CEKATID-^: . 
29 
Trochoceras arietinum is distinguished from T. rapacc by the pro- 
portions of its whorls and its more delicate ribbing. 
Horizon. Wenlock Shales, Lower Ludlow. 
Locality. Llangollen, Denbighshire (W. S.) ; Ledbury, Hereford- 
shire (L.-L.). 
Represented in the Collection by two examples, one of which 
(Xo. C. 1998) was presented by J. E. Lee, Esq., F.G.S. 
Trochoceras equisetum, Blake, sp. 
1839. Phraymoceras? nautileum (pars), J. de C. Sowerby, in Murchi- 
son’s Sil. Syst. pt. ii. p. 622, pi. x. f. 2 {not f. 3). 
1882. Cyrioceras {?) equisetum, Blake, British Foss. Ceph. pt. i. p. 181, 
pi. XXX. f. 7. 
Char. The section is doubtful, but at present the surface of 
the side exposed is uniformly convex. The curvature is very great, 
the mean radius being 1| inches where the diameter of the whorl is 
about l;j inches. Thus, though the specimen is imperfect at the 
smaller end and there arc no signs of contact of an earlier whorl, 
the appearances are almost those of a coiled shell. The rate of 
increase is about 1 in 15, measured along the outer curve. The 
ornaments are very rough irregular risings, curving rapidly back- 
wards from the inner edge, and covered with parallel irregular 
lines of growth. Possibly 3 inches of mean length belong to the 
body-chamber. There is no sign of any change towards the aper- 
ture, which will be similar to the general ribbing, and therefore 
have a deep sinus on the front. The septa are rather remote, being 
from 5 to k the present diameter of the whorl apart, or 9 in the 
quarter circumference seen. The sutures curve rapidly forwards 
towards the convex side, cutting across several lines of ornaments. 
The siphuncle is external. Its elements are much narrower than 
their length, and have a conical shape, enlarging towards the aper- 
ture, and suddenly decreasing on passing each septum. The 
diameter across the shell is about 3| inches, and the greate.st 
diameter of the whorl is 1^ inches.” This is Professor Blake’s 
description of the specimen which he selected as the typo of his 
species. He thus remarks upon its affinities : — “ If this be a 
Cyrtoceras. there is very little doubt of its distinctness from all 
others ; none of those showing so much curvature being anything 
like it. On the other hand, if it be a Trochoceras, its peculiar 
siphuncle, combined with the irregularity of its ribbing, separate it 
well from all those in which those characters are known.” 
Remarks. Of the two interpretations given by Professor Blake 
