ensis_ (fig. 144, b);
the small Cockle, _Cardium Rhoeticum_ (fig. 144, d); and the
curiously-twisted Pearl-oyster, _Avicula contorta_ (fig. 144,
e)--this last Bivalve being so abundant that the strata in
question are often spoken of as the "Avicula contorta beds."

[Illustration: Fig. 145.--_Ceratites nodosus_, viewed from the
side and from behind. Muschelkalk.]

Passing over the groups of the _Heteropods_ and _Pteropods_, we
have to notice the _Cephalopoda_, which are represented in the
Trias not only by the chambered shells of _Tetrabranchiates_, but
also, for the first time, by the internal skeletons of _Dibranchiate_
forms. The Trias, therefore, marks the first recognised appearance
of true Cuttle-fishes. All the known examples of these belong
to the great Mesozoic group of the _Belemnitidoe_; and as this
family is much more largely developed in the succeeding Jurassic
period, the consideration of its characters will be deferred till
that formation is treated of. Amongst the chambered _Cephalopods_
we find quite a number of the Palaeozoic _Orthoceratites_, some of
them of considerable size, along with the ancient _Cyrtoceras_
and _Goniatites_; and these old types, singularly enough, occur
in the higher portion of the Trias (St Cassian beds), but have,
for some unexplained reason, not yet been recognised in the lower
and equally fossiliferous formation of the Muschelkalk. 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 Tri