114 
logy authorizes the belief that such is sometimes their natural form ; 
since this form is observable in many of the different species of the 
minute recent concamerated shells. 
In none of the multilocular shells are such differences observable, 
with respect to the size of the siphunculi, as in this genus. In some 
the siphunculus does not equal one tenth, whilst in others it exceeds a 
third, and is sometimes nearly one half, of the diameter of the orthoce- 
ratite itself. It generally assumes that tumid form, which a membra- 
neous part might be expected to assume, when dilated between the 
septa : as may be seen in the large siphunculus, Plate VIII. Fig. 2, and 
in Fig. /% where the siphunculus is seen intersected by the septa of the 
orthoceratites ; the whole being imbedded in a mass of whitish marble. 
The siphunculus, or rather the cast of the siphunculus, of this shell, 
very often yields a striking appearance, from an obliquity in its form, 
as in Plate VIII. Fig. 2, and Fig. 6. This appearance has contri- 
buted somewhat to the degree of error which has existed, respecting 
the original nature of these bodies ; since, even those who might be 
disposed to consider them as deriving their form from the siphun- 
culus of an orthoceratites, might expect to find a corresponding ob- 
liquity of form in the orthoceratites itself. But a slight investigation 
will show, that there exists no reason for supposing, from this cir- 
cumstance, that the shell to which it belonged possessed any parti- 
cular degree of obliquity of form. 
The septa, by which the siphunculus is intersected, being a segment 
of a hollow sphere, the divisions of the siphunculus must necessarily 
vary with the direction with which the siphunculus passes through 
them. If the siphunculus passes through the centre of the septa, and 
in a line nearly parallel with the parieties of the tube, it will be marked 
by segments of a hollow sphere, the sections of which are disposed in 
nearly transverse lines, as at Plate VIII. Fig. 3. But if the siphun- 
culus does not pass through the centre of the septa, and yet keeps a 
line nearly parallel with the parieties of the tube, it will be marked 
