134 
NAUTILOIDEA. 
animals. Its siphuncle is mostly situated quite on the margin 
or on the circumference of the shell, and not quite hidden by the 
joints of the external shell ; occupying about a third part of the 
entire shell. On account of its situation on the margin, the segmental 
partitions [septa] of the shell form very oblique rings on the sur- 
face of the siphuncle, and at the same time run outwards over its 
face, whence it results that the siphuncle seems as it were to consist 
of hoods or wrappers, set or inserted upon one another, as if it 
sent out processes. The outer wall of the siphuncle is entire 
and free from perforations, showing no point of communication 
between the cavity of the siphuncle and the chambers of the 
exterior shell.” 
The author then proceeds to describe the smaller Ortlioceras 
lodged in the siphuncle of “ Ortlioceras duplex^' from which 
circumstance the specific name originated. It need hardly be 
said that the included Ortlioceras was introduced by accident 
into the capacious siphuncle of the larger one’. Judging by 
noid, AulxLcoceras, shows that the secreting-organ of the guard must have been 
open, and proves it by his drawings of masses of foreign matter included in 
the layers of the guard. This and the channel which runs along the ventral 
side in the guard and is often single, and generally more persistent and 
longer than the dorsal channel, favour this view of the homologies. Queustedt 
and von Ihering have both traced the Belemnoids to Orthoceras ; and this 
opinion supports their views and accounts for the preservation of the proto- 
conch [imtial chamber], both as a useful organ containing the siphonal cjecum 
in its neck, and because of the protective guard built up around it at an early 
stage of growth. The tracing of Belemnoidea back to Orthoceras accounts for 
the dorsal and ventral channels, both of which are present in the plug of the 
truncated Orthoceras. It also permits us to explain the central hollow trace 
or tube in the guard of the Belemnoid as the homologue of a similar hollow 
trace or tube, the pseudo -siphuncle, which invariably occupies the centre of 
the plug in trimcated Orthoceratites, and has not been heretofore accounted 
for.” 
Prof. Hyatt’s explanation of the manner in which the conical cap was 
formed is not quite satisfactory, because if the extremity of the shell became 
periodically truncated, the calcareous deposit at its apex could not have been 
renewed each time by the mantle-fold, unless the shell were internal, or at least 
partly so, in order that its apex might be reached by the shell-secreting part of 
the animal’s body. It is different with the shells in which no truncation has 
taken place, for in them the apical deposit was made at a very early stage of 
growth, when the shell was in immediate contact with the animal. 
^ This is, of course, no uncommon thing. Barrande gives numerous figures 
of Endoceras and Orthoceras into whose siphuncles yoimg or smaller shells 
belonging to those genera have been introduced after the death of the animal 
and the partial destruction of its shell. (See Syst. Sil. de la Boheme, vol. ii. 
jd. ccccxxxviii. figs. 9, 11 ; also Pal. of Hew York, by James Hall, toI. i. 1847, 
pi. xlviii. fig. 3.) 
