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 Palæozoic _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. 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 Palæozoic and Mesozoic types which they exhibit. It is to be remembered, also, that this intermixture has hitherto been recognised, not in the Middle Triassic limestones of the Muschelkalk, in which--as the oldest Triassic beds with marine fossils--we should naturally expect to find it, but in the St Cassian beds, the age of which is considerably later than that of the Muschelkalk. The intermingling of old and new types of Shell-fish in the Upper Trias is well brought out in the annexed table, given by Sir Charles Lyell in his 'Student's Elements of Geology' (some of the less important forms in the table being omitted here):--