ACTIXOCERATID^. 
183 
1878. OHhoceras nummularium, Etheridge, Quart. Journ. Geol. Soc. 
Tol. xxxiv. p. 608. {Not of SoAverby.) 
[iVo^ 1868. Ortlioceras (Ormoceras) Backii, Meek and Worth en, Geol. 
Sui’v. of Illinois, vol. hi. p. 298, pi. i. f. 4. — 1870. OHhoceras 
Backi, Barrande, Sjst. Sil. de la Boheme, toI. u. Texte hi. 1874, 
p. 731, pi. ccccxxxvh. ff, 17, 18.] 
Bip. Char. Stokes’s description is much too brief to be serviceable, 
consisting only of the words, “ siphuncle larger than the preceding ” 
[i. e. A. Bayjieldi], and a reference to plate vi. in Breyniiis’s Dissert, 
phys. de Polythalam. I therefore give Bhlings’s description, which 
was drawn up from more perfect specimens, and enables one to make 
out the characters of the species with tolerable accuracy : — “ Septa 
very convex, four lines distant at a diameter of one inch eleven lines ; 
siphuncle large, dilated between the septa, constituting an obliquely 
nummuloid cylinder fourteen hues in diameter, where the diameter 
of the perfect fossil was two inches, and situated within three lines 
of the ventral margin. The annulations have an obliquity of about 
20° to the longitudinal axis, and they are evenly convex from one 
septum to another.” 
Bemarks. The type specimen from Drummond Island is silicified, 
the walls of the siphuncle, of which five or six segments are pre- 
served, having been replaced by orbicular chalcedony (“ beekite ”). 
A perforation is seen at both extremities of the siphuncle indicating 
the position of the endosiphon, and the tubuli given off b}' that organ 
are represented by a series of irregular horizontal cracks in the most 
prominent part of the siphuncle. 
Two fragments, collected in the Arctic Eegions during the expe- 
dition of the ‘ Alert ’ and ‘ Discovery,’ are included prcJVisionally 
in this species, but their condition is such as to make it impossible 
to do more than suggest their affinity with it. 
Horizon. Niagara Group (Wenlock). 
Localities. Cape Louis Napoleon, Smith Sound; Bessels Bay, 
Kennedy Channel ; Arctic America. Drummond Island, Lake Huron ; 
Canada (exact locality unknown). 
Bepresented by the type specimen, which was presented to the 
Museum by Dr. J. J. Bigsby, F.R.S. The Arctic specimens, of which 
there are three, were transferred from the Museum of Practical 
Geology. There are two other specimens besides these, so that the 
species may be said to be well illustrated. 
