14 CLASS CEPHALOPODA. 
the whorls are sometimes contiguous and sometimes distinct. 
The HorTo Les of Montfort. 
In others, the orthoceratites, it is altogether straight ; it is 
not improbable that the animal resembled that of the nautilus, 
or of the spirula. 'The 
BELEMNITES 
Probably belong to this family ; but it is impossible to ascer- 
tain the fact, as they are only found among fossils. Every 
thing, however, proves them to have been internal shells, thin, 
and double; that is, composed of two cones united at base, 
the inner one much shorter than the other, and divided into 
chambers by parallel septa, which are concave on the side 
next to the base. A siphon extends from the summit of the 
external cone to that of the internal one, and continues thence, 
sometimes along the margin of the septa, and sometimes 
through their centre. The interval between the two testace- 
ous cones is filled with a solid substance, here composed of 
radiating fibres, and there of self-involving conical layers, the 
base of each being on the margin of one of the septa of the 
inner cone. In one specimen we only find this hard portion, 
and in another we also find the nuclei of the chambers of the 
inner cone, or what are termed the alveoli. Most commonly 
these nuclei and the chambers themselves have left no further 
traces than some projecting circles on the inside of the internal 
cone. In other specimens, again, we find more or fewer of the 
nuclei, and still in piles, but detached from the double conical 
sheath that enveloped them. 
Of all the fossils the Belemnites are the most abundant, 
particularly in chalk and compact limestone. 
M. de Blainville divides them according as the interior 
cone or chambered part penetrates to a greater or less depth, 
as the edges of the exterior cone have or have nota little cleft, 
as the external surface is marked on one side by a longitudi- 
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