Along
with these we meet for the first time with true _Ammonites_,
which fill such an extensive place in the Jurassic seas, and
which will be spoken of hereafter. The form, however, which is
most characteristic of the Trias is _Ceratites_ (fig. 145). In
this genus the shell is curved into a flat spiral, the volutions of
which are in contact; and it further agrees with both _Goniatites_
and _Ammonites_ in the fact that the septa or partitions between
the air-chambers are not simple and plain (as in the _Nautilus_
and its allies), but are folded and bent as they approach the
outer wall of the shell. In the _Goniatite_ these foldings of
the septa are of a simply lobed or angulated nature, and in the
_Ammonite_ they are extremely complex; whilst in the _Ceratite_
there is an intermediate state of things, the special feature
of which is, that those foldings which are turned towards the
mouth of the shell are merely rounded, whereas those which are
turned away from the mouth are characteristically toothed. The
genus _Ceratites_, though principally Triassic, has recently
been recognised in strata of Carboniferous age in India.
From the foregoing it will be gathered that one of the most important
points in connection with the Triassic _Mollusca_ is the remarkable
intermixture of Palaeozoic and Mesozoic types which they exhibit.
Pages:
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328