ENDOCERATID^. 
133 
“ Orthoceras duplex,” Wahlenberg et auctt. — There are a great 
number of specimens of Endoceras in the Collection from Sweden, 
Russia, and Xortbern Germany, bearing the well-known name of 
“Orthoceras duplex,” AVablenberg. On turning, however, to the 
original description (Xova Acta Reg. Soc. Scient. Rpsaliensis, 1821, 
vol. viii. p. 86) it is found to be quite insufficient for purposes of 
identification, and has led to many diverse forms being united 
under TTablenberg’s appellation. The essential part of the de- 
scription is contained in the following passage, translated from the 
Latin original : — 
“ Orthoceratites duplex or gigauteus is found in Westrogotbia, and 
in this district only are complete and well developed specimens met 
with, particularly in Mt. Kinnekulle. The diameter is mostly a 
hand’s breadth, and sometimes as much as five Parisian inches, a 
size of the tube surpassing that of any other univalved many- 
chambered testaceous animal which I have seen alive or dead. Its 
form is rather cylindrical than conical, and in addition to its great 
breadth it has a length of six feet and more, so that we are con- 
vinced that it surpasses in magnitude all Ammonites hitherto dis- 
covered, and that it is thus the largest of all univalved testaceous 
the plug \^= calotte conique of Rarrande] which fills up and rounds off the 
broken apex, accounted for as an extei'nal product due probably to the action 
of two arms which he supposed stretched out posteriorly from the aperture, 
as in Argonauta, and embracing the whole shell, and extending even beyond 
the broken end, filled it up by successive deposits of carbonate of lime. We 
have also studied this structure, and can confirm M. Barrande’s observation, 
and his conclusion that the plug is an external product with a peculiar bilateral 
structure ; but that it was deposited by two arms extending backwards we do 
not regard as probable. The markings indicate that at the truncated apex the 
layers were laid on by an organ which was certainly double at the end, but not 
necessarily double all the way up to the aperture. The body of the whorl [i. e. 
the whole of the shell above the conical cap] in Orthoceras does not have 
any prolongations of the ventral and dorsal furrows of the plug, nor are there 
any longitudinal rows of spines or furrows, as in Argonauta, indicating the 
presence of two arms, capable of secreting shell, nor does the aperture with its 
entire outline give strong support to this proposition. 
“ We think it is possible to strike nearer home in homology; the fold of the 
mantle in Nautilus satisfies the conditions. This is an active shell-secreting 
organ, which was certainly present, and also functionally active, in the Ammo- 
noidea and Nantiloids, and probably more important in these ancient forms 
than it is now in the modern Nautilus. This is also more consistent with the 
explanation of the structure of the Belemnoid, which, as is easily seen in the 
famous examples of the preserved animal, had no such pair of enlarged arms, 
and yet deposited externally a solid covering — the guard — which is in our 
opinion the homologue of the solid filling of the truncated end of Orthoceras. 
Branco, in his admirable paper on the structure of the curious Triassic Belem- 
