THE JURASSIC PERIOD. 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 oi 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 Ammonitidce. 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 i^Beloteuthis subcosfata, fig. 

 172) belonged to an old type essentially similar to our modern 

 Calamaries, the skeleton of which consists of a horny shaft 



