Fig. 144. Triassic Lamellibranchs. a, Daonella (Halobia) Lommelli; b, Pecten Valoniensis; c, Myophoria lineata; d. Cardium Rhœticum; e. Avicula contorta; f. Avicula socialis. abundant, and are found in the Triassic strata of almost all regions. These groups belong to the family of the Pearl-oysters (Aviculidœ), and are singular from the striking resemblance borne by some of their included forms to the Strophomenœ amongst the Lamp-shells, though, of course, no real relation exists between the two. The little Pearl-oyster, Avicula socialis (fig. 144, f), is found throughout the greater part of the Triassic series, and is especially abundant in the Muschelkalk. The genus Myophoria (fig. 144, c), belonging to the Trigoniadœ, 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 "Rhætic" or "Kössen" beds are characterised by the occurrence in them of the Scallop, Pecten Valoniensis (fig. 144, b); the small Cockle, Cardium Rhœticum (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."
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 Belemnitidœ; 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
Fig. 145.—Ceratites nodosus, viewed from the side and from behind. Muschelkalk. 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):—
GENERA OF FOSSIL MOLLUSCA IN THE ST CASSIAN AND HALLSTADT BEDS.
| Common to Older Rocks. | Characteristic of Triassic Rocks. | Common to Newer Rocks. | ||||||||||||||||||||||||||||||||||
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Thus, to emphasise the more important points alone, the Trias has yielded, amongst the Gasteropods, the characteristically Palæozoic Loxonema, Holopella, Murchisonia, Euomphalus, and Porcellia, along with typically Triassic forms like Platystoma and Scoliostoma, and the great modern groups Chemnitzia and Cerithium. Amongst the Bivalves we find the Palæozoic Megalodon side by side with the Triassic Halobia and Myophoria, these being associated with the Carditœ, Hinnites, Plicatulœ, and Trigoniœ of later deposits. The Brachiopods exhibit the Palæozoic Athyris, Retzia, and Cyrtina, with the Triassic Koninckia and the modern Thecidium. Finally, it is here that the ancient genera Orthoceras, Cyrtoceras, and Goniatites make their last appearance upon the scene of life, the place of the last of these being taken by the more complex and almost exclusively Triassic Ceratites, whilst the still more complex genus Ammonites first appears here in force, and is never again wanting till we reach the close of the Mesozoic period. The first representatives of the great Secondary family of the Belemnites are also recorded from this horizon.
Amongst the Vertebrate Animals of the Trias, the Fishes are represented by numerous forms belonging to the Ganoids and the Placoids. The Ganoids of the period are still all provided with unsymmetrical ("heterocercal") tails, and belong principally to such genera as Palœoniscus and Catopterus. The remains of Placoids are in the form of teeth and spines, the two principal genera being the two important Secondary groups Acrodus and Hybodus. Very nearly at the summit of the Trias in England, in the Rhætic series, is a singular stratum, which is well known as the "bone-bed," from the number of fish-remains which it contains. More interesting, however, than the above, are the curious palate-teeth of the Trias, upon which Agassiz founded the genus Ceratodus. The teeth of Ceratodus (fig. 146) are singular flattened plates, composed