Upper Silurian. PALAONTOLOGY OF VICTORIA. { Mollusca. 
PP if 
Pirate LVII., Fias. 1 anp 2. 
ORTHOCERAS (CYCLOCERAS) IBEX (Sow.). 
[Genus ORTHOCERAS (Breyy.) restricted. (Sub-kingd. Mollusca, Class Cephalopoda. 
Order Tetrabranchiata, Fam. Nautilide.) 
Gen. Char.—Shell conical, straight or nearly so, having the greater part of the posterior 
end traversed by convex, transverse septa, with simple edges, at right angles to the long axis of 
the shell; siphon calcareous, central or slightly excentric, cylindrical or dilated between the 
chambers ; external surface even. 
Sub-genus Lococeras (McCoy). General form of Orthoceras proper, but the septa are 
oblique to the long axis, and have a deep wave in their edges on each side, and the siphon is 
either marginal or submarginal, varying, however, as in Orthoceras, in thickness or inflation 
between the septa. 
Sub-genus Cycloceras (McCoy). Shell straight, or slightly curved at the tip, tapering ; 
section circular or slightly oval; girt with prominent transverse rings; siphon varying from 
sub-central to marginal ; septa at right angles to the axis of the shell, with even edges ; surface 
often sculptured with scaly lamin, or decussated. 
The external characters of this ringed and often sculptured group is so strongly marked 
that it forms a useful, easily recognised sub-genus, provisionally, apart from the true 
Orthoceras. | 
DescRipTion.—Shell long, slightly compressed, nearly cylindrical (tapering at 
the rate of about half a line in 3 inches at the diameter of 6 inches), with occasional 
abrupt diminutions of diameter; girt with prominent, narrow, rounded, slightly 
oblique, and very slightly flexuous rings, about 3 of a line thick, separated by rather 
wider concave spaces; about five rings in a space of 6 lines at 6 lines in diameter; 
septa, one between each pair of rings; section broad oval; siphon slightly excentric 
towards one of the broad sides. Surface with very fine longitudinal furrows and 
close transverse strie. 
REFERENCE. = O. Ibex (Sow.) + O. articulatum (Sow.); Murch. Sil. Syst., t. 5, 
fig. 30, 31. 
On the most careful comparison with English and Swedish 
Upper Silurian specimens I cannot find any difference in the 
Australian examples. As in the English specimens when preserved 
like ours in sandstone it is extremely difficult to detect traces of 
the striation, but they may be seen by those who know what to 
look for. Although Mr. Sowerby does not mention the transverse 
striation, Mr. Salter (who is understood to have engraved the 
figure) distinctly states that it is present in the original specimen 
of the above quoted, t. 5, fig. 31, in his remarks at page 354 of 
the pt. 1, vol. ii., of the Memoirs of the Geological Survey of the 
United Kingdom, and I have mentioned observing the same char- 
acter in other specimens from the Upper Ludlow rocks of West- 
DEC. V1. [ 25 ] D 
