THE PERMIAN PERIOD. 



199 



feebly represented at the present day, is one which attained its 

 maximum development in the Mesozoic period. 



The Univalves (Gasteropoda} are rare, and do not demand 

 special notice. It may be ob- 

 served, however, that the Palaeo- 

 zoi u genera Euomphahis, Mur- 

 chisonia, Loxonema, and Macro- 

 chtilus are still in existence, to- 

 gether with the persistent genus 

 Pleurotomaria. Pteropods of the 

 old genera Theca and Conula- 

 ria have been discovered; but 

 the first of these characteristi- 

 cally Palaeozoic types finally 



dies out here, and the second F ; g . i 3 6.-/im /,*** Permian, 

 only survives but a short time 



longer. Lastly, a few Cephalopods have been found, still wholly 

 referable to the Tetrabranchiate group, and belonging to the old 

 genera Orthoceras and Cyrtoceras and the long-lived Nautilus. 



Amongst Vertebrates, we meet in the Permian period not 

 only with the remains of Fishes and Amphibians, but also, for 

 the first time, with true Reptiles. The Fishes are mainly 

 Ganoids, though there are also -emains of a few Cestraciont 



Fig. 137 Platysomus zibbosns, a "heterocercal" Ga 

 Middle Permian of Russia. 



Sharks. Not only are the Ganoids still the predominant group 

 of Fishes, but all the known forms possess the unsymmetrical 

 ("heterocercal") tail which is so characteristic of the Palaeozoic 

 Ganoids. Most of the remains of the Permian Fishes have 

 been obtained from the " Marl-slate " of Durham and the 

 corresponding " Kupfer-schiefer " of Germany, on the horizon 



