Within these limits, each formation is characterised by particular species, the number of individuals being often very great, 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 type 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