THE TRIASSIC PERIOD. 



213 



tremely complex ; whilst in the Ceratite there is an inter- 

 mediate 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 Ceratifes, 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 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 Older Rocks. 



Orthoceras. 



Bactrites. 



Macrocheilus. 



Loxonema. 



Holopella. 



Murchisonia. 



Porcellia. 



Athyris. 



Retzia. 



Cyrtina. 



Euomphalus. 



Characteristic of Triassic 

 Rocks. 



Ceratites. 



Cochloceras. 



Rhabdoceras. 



Aulacoceras. 



Naticella. 



Platystoma. 



Halobia. 



Hornesia. 



Koninckia. 



Scoliostoma. 



Myophoria. 

 (The last two are princi- 

 pally but not exclus- 

 ively Triassic. ) 



Common to Newer Rocks. 



Ammonites. 



Chemnitzia. 



Cerithium. 



Monodonta. 



Sphoera. 



Cardita. 



Myoconcha. 



Hinnites. 



Monotis. 



Plicatula. 



Pachyrisma. 



Thecidium. 



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 Myo- 

 phoria, these being associated with the Cardites, Hinnites^ 

 Plicatulce^ and Trigonice of later deposits. The Brachiopods 



