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 Palaeozoic 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       |   Characteristic of  |    Common to
Older Rocks.    |   Triassic Rocks     |    Newer Rocks.
|                      |
Orthoceras.     |    Ceratites.        |    Ammonites. 
Bactrites.      |    Cochloceras.      |    Chemnitzia. 
Macrocheilus.   |    Rhabdoceras.      |    Cerithium. 
Loxonema.       |    Aulacoceras.      |    Monodonta. 
Holopella.      |    Naticella.        |    Sphoera. 
Murchisonia.    |    Platystoma.       |    Cardita. 
Porcellia.      |    Halobia.          |    Myoconcha. 
Athyris.        |    Hoernesia.         |    Hinnites. 
Retzia.         |    Koninckia.        |    Monotis. 
Cyrtina.        |    Scoliostoma.      |    Plicatula. 
Euomphalus.     |    Myophoria.        |    Pachyrisma.
|(The last two are     |    Thecidium.
|principally but not   |
|exclusively Triassic.)|

Thus, to emphasise the more important points alone, the Trias has yielded, amongst the Gasteropods, the characteristically Palaeozoic 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 Palaeozoic Megalodon side by side with the Triassic Halobia and Myophoria, these being associated with the Carditoe, Hinnites, Plicatuloe, and Trigonioe of later deposits.  The Brachiopods exhibit the Palaeozoic 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.