and the size which is sometimes attained being nothing short
of gigantic. In the Lias, particular species of _Ammonites_ may
succeed one another regularly, each having a more or less definite
horizon, which it does not transgress. It is thus possible to
distinguish a certain number of zones, each characterised by
a particular Ammonite, together with other associated fossils.
Some of these zones are very persistent and extend over very
wide areas, thus affording valuable aid to the geologist in his
determination of rocks. It is to be remembered, however, that
there are other species which are not thus restricted in their
vertical range, even in the same formations in which definite
zones occur.

[Illustartion: Fig. 172.--_Beloteuthis subcostata_ Jurassic (Lias).]

The Cuttle-fishes or _Dibranchiate Cephalopods_ constitute a
feature in the life of the Jurassic period little less conspicuous
and striking than that afforded by the multitudinous and varied
chambered shells of the _Ammonitidoe_. The remains by which these
animals are recognised are necessarily less perfect, as a rule,
than those of the latter, as no external shell is present (except
in rare and more modern groups), and the internal skeleton is
not necessarily calcareous. Nevertheless, we have an ample record
of the Cuttle-fishes of the Jurassic period, in the shape of
the fossilised jaws or beak, the ink-bag, and, most commonly
of all, the horny or calcareous structure which is embedded in
the soft tissues, and is variously known as the "pen" or "bone."
The beaks of Cuttle-fishes, though not abundant, are sufficiently
plentiful to have earned for themselves the general title of
"Rhyncholites;" and in their form and function they resemble
the horny, parrot-like beak of the existing Cephalopods. The
ink-bag or leathery sac in which the Cuttle-fishes store up the
black pigment with which they obscure the water when attacked,
owes its preservation to the fact that the colouring-matter which
it contains is finely-divided carbon, and therefore nearly
indestructible except by heat. Many of these ink-bags have been
found in the Lias; and the colouring-matter is sometimes so well
preserved that it has been, as an experiment, employed in painting
as a fossil "sepia." The "pens" of the Cuttle-fishes are not
commonly preserved, owing to their horny consistence, but they
are not unknown. The form here figured (_Beloteuthis subcostata_,
fig. 172) belonged to an old typ