246 HISTORICAL PALEONTOLOGY. 



take place, that the edges of the septa, when exposed by the 

 removal of the shell-substance, present in an exaggerated man- 

 ner the appearance exhibited by an elaborately-dressed shirt- 

 frill when viewed edgewise. The species of Ammonites range 

 from the Carboniferous to the Chalk; but they have not been 

 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 characterized 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 characterized by a particular Ammonite, 

 together with other associated fossils. Some of these zones 

 are very persistent and extend over very wide areas, thus af- 

 fording 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 Ammonitidtz. The remains by which 

 these animals are recognized 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 fossilized 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 coloring-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 



