THE TRIASSIC PERIOD. 



219 



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 Bocks. 



Orthoceras. 



Bactrites. 



Macrocheilus. 



Loxonema. 



Holopella. 



Murchisonia. 



Porcellia. 



Athyris. 



Retzia. 



Cyrtina. 



Euomphalus. 



Thus, to emphasize the more important points alone, the Trias 

 has yielded, amongst the Gasteropods, the characteristically 

 Palaeozoic Loxoncma, 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 

 Mcgalodon side by side with the Triassic Halobia and Myo- 

 phoria, these being associated with the Cardita, Hinnites, 

 Plicatulce, and Trigonia 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 Gonia- 

 tites 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 



