Lastly, the so-called
"Rhaetic" or "Koessen" beds are characterised by the occurrence
in them of the Scallop, _Pecten Valoniensis_ (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.
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