THE JURASSIC. BERIOD: 239 
found in deposits older than the Secondary, in any region 
except India; and they are therefore to be regarded as essen- 
tially Mesozoic fossils. Within these limits, each formation 
is characterised by particular species, the number of individ- 
uals being often very great, and the size which is sometimes 
attained being nothing short of gigantic. In the Lias, par- 
ticular species of Ammonites may succeed one another regu- 
larly, 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 afford- 
ing 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. 
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 Ammonitide. ‘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 them- 
selves 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-divid- 
ed 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 (Lelofeuthis subcostata, fig. 
172) belonged to an old type essentially similar to our modern 
Calamaries, the skeleton of which consists of a horny shaft 
