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. | Hörnesia. | 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 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 _Carditoe, Hinnites, Plicatuloe_, and _Trigonioe_ 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. [Illustration: Fig. 146.--a, Dental plate of _Ceratodus serratus_, Keuper; b, Dental plate of _Ceratodus altus_, Keuper; (After Agassiz.)] [Illustration: Fig 147.--_Ceratodus Fosteri_, the Australian Mud-fish, reduced in size.]