abundant in the Muschelkalk.  The genus Myophoria (fig. 144, c), belonging to the Trigoniadoe, and related therefore to the Permian Schizodus, is characteristically Triassic, many species of the genus being known in deposits of this age.  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.  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.