MOLLUSCA. GASTEROPODA. 167 



somewhat doubtful; it is possible that they may not be 

 Pteropods; but representatives of the order certainly occur 

 in the Silurian and Devonian. 



The most important genera found in the different 

 systems are : 



Cambrian. Bellerophon, Hyolithes, Conularia. 



Ordovician. Bellerophon, Holopcea, Cyclonema, Mac- 

 lurea, Conularia, Tentaculites. 



Silurian. Pleurotomaria, Murchisonia, Bellerophon, 

 Euomphalus, Holopella, Acroculia. 



Devonian. Pleurotomaria, Murchisonia, Bellerophon, 

 Loxonema, Euomphalus, Macrocheilus, Tentaculites. 



Carboniferous. Pleurotomaria, Murchisonia, Bellero- 

 phon, Loxonema, Euomphalus, Naticopsis, Macrocheilus, 

 Conularia. 



Permian. Pleurotomaria, Murchisonia, Turbo, Loxo- 

 nema, Macrocheilus. 



Trias. Pleurotomaria, Turbo, Loxonema, Natica, Turri- 

 tella. 



Jurassic. Pleurotomaria, Amberleya, Cirrus, Phasia- 

 nella, Trochus, Natica, Pseudomelania, Nerinea, Cerithium, 

 Alaria, Purpuroidea. 



Cretaceous. Pleurotomaria, Solarium, Turritella, Na- 

 tica, Paludina, Cerithium, Aporrhais, Cinulia. 



Eocene. Phorus, Calyptrcea, Natica, Melania, Ceri- 

 thium, Rostellaria, Cyprcea, Cassidaria, Triton, Fusus, 

 Pyrula, Murex, Typhis, Voluta, Oliva, Pleurotoma, 

 Conus. 



Oligocene. Neritina, Natica, Paludina, Melania, Ceri- 

 thium, Potamides, Murex, Voluta, Ancillaria, Pleurotoma, 

 Limncea, Planorbis, Rissoa, Helix, Bulimus. 



