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.
Pages:
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363