e essentially similar to our modern
Calamaries, the skeleton of which consists of a horny shaft and
two lateral wings, somewhat like a feather in general shape. When,
on the other hand, the internal skeleton is calcareous, then it is
very easily preserved in a fossil condition; and the abundance of
remains of this nature in the Secondary rocks, combined with their
apparent total absence in Palaeozoic strata, is a strong presumption
in favour of the view that the order of the Cuttle-fishes did
not come into existence till the commencement of the Mesozoic
period. The great majority of the skeletons of this kind which are
found in the Jurassic rocks belong to the great extinct family
of the "Belemnites" (_Belemnitidoa_), which, so far as known, is
entirely confined to rocks of Secondary age. From its pointed,
generally cylindro-conical form, the skeleton of the Belemnite is
popularly known as a "thunderbolt". (fig. 173, C). In its perfect
condition--in which it is, however, rarely obtainable--the skeleton
consists of a chambered conical shell (the "phragmacone"), the
partitions between the chambers of which are pierced by a marginal
tube or "siphuncle." This conical shell--curiously similar in its
structure to the _external_ shell of the Nautilus--is extended
forwards into a horny "pen," and is sunk in a corresponding conical
pit (fig. 173, B), excavated in the substance of a nearly cylindrical
fibrous body or "guard," which projects backwards for a longer or
shorter distance, and is the part most usually found in a fossil
condition. Many different kinds of _Belemnites_ are known, and
their guards literally swarm in many parts of the Jurassic series,
whilst some specimens attain very considerable dimensions. Not
only is the internal skeleton known, but specimens of _Belemnites_
and the nearly allied _Belemnoteuthis_ have been found in some
of the fine-grained sediments of the Jurassic formation, from
which much has been learnt even as to the anatomy of the soft
parts of the animal. Thus we know that the Belemnites were in
many respects comparable with the existing Calamaries or Squids,
the body being furnished with lateral fins, and the head carrying
a circle of ten "arms," two of which were longer than the others
(fig. 173, A). The suckers on the arms were provided, further,
with horny hooks; there was a large ink-sac; and the mouth was
armed with horny mandibles resembling in shape the beak of a
parrot.

[Illustration