THE PERMIAN PERIOD. 



203 



development in the Permian rocks; but nearly all the old 

 types, except some of those which occur in the Carboniferous, 

 have now disappeared. The principal Permian Bivalves 

 belong to the groups of the Pearl Oysters (Aviculida) and the 

 Trigoniadcu, represented by genera such as Bakewellia and 

 Schisodus; the true Mussels (Mytilida), represented by species 

 which have been referred to Mytilus itself; and the Arks 

 (Arcades}, represented .by species of the genera Area (fig. 136) 

 and Byssoarca. The first and last of these three families have 

 a very ancient origin ; but the family of the Trigoniadce, though 

 feebly represented at the present day, is one which attains 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- 

 zoic genera Euomphalus, Mur- 

 chisonia, Loxonema, and 

 Macrocheilus are still in exist- 

 ence, together with the per- 

 sistent genus Pleurotomaria. 

 Pteropods of the old genera 

 Theca and Conularia have been 

 discovered ; but the first of 

 these characteristically Palaeo- 

 zoic types finally dies out here, Fig. 136. Area antiqua. Permian, 

 and the second only survives 

 but a short time longer. Lastly, a few Cephalopods have been 



Fig. 137. I'latysoums gibbosua, a " heterocercal " Ganoid, from the 

 Middle Permian of Russia. 



found, still wholly referable to the Tetrabranchiate group, and 



