254 MOLLUSCA. GASTEROPODA 



Gasteropoda are not nearly so well represented as the 

 Streptoneura. The Isopleura range from the Ordovician 

 to the present day, but are rare as fossils. The Heteropoda 

 are represented by a few forms only, the first occurring 

 in the Miocene. The Opisthobranchia range from the 

 Carboniferous to the present day ; they are moderately well 

 represented in the Jurassic and Cretaceous formations, and 

 become more abundant in the Tertiary. Pteropods are 

 found in the Upper Cretaceous and later formations. 

 Marine forms of Pulmonata appear first in the Devonian ; 

 non-marine forms are found in the Carboniferous, but are 

 rare until the Purbeck and Wealden periods, and become 

 abundant in the Tertiary deposits. 



The most important genera of Gasteropoda found in 

 the different systems are : — 



Cambrian. Scenella, Platyceras, Raphistoma, Bellerophon, Stra- 

 parollina, Ophileta, Stenotheca. 



Ordovician. Bellerophon, Holopcea, Cyclonema, Maclurea. 



Silurian. Pleurotomaria, Bellerophon, Omphalotrochus, Holopcea, 

 Cyclonema, Holopella, Platyceras. 



Devonian. Pleurotomaria, Murchisonia, Bellerophon, Loxonema, 

 Euomphalus, Macrochilina, Capulus. 



Carboniferous. Metoptoma, Pleurotomaria, Murchisonia, Beller- 

 ophon, Loxonema, Euomphalus, Naticopsis, Macrochilina, Capulus. 



Permian. Pleurotomaria, Murchisonia, Loxonema, Macrochilina. 



Trias. Pleurotomaria, Trochus, Loxonema, Scala, Naticopsis, 

 Natica, Turritella. 



Jurassic. Pleurotomaria, Amberleya, Cirrus, Trochus, Natica, 

 Pseudomelania, Bourguetia, Nerinea, Cerithium, Dicroloma 

 ( = Alaria), Malaptera, Purpurina, Purpuroidea. 



Cretaceous. Pleurotomaria, Solarium, Turritella, Natica, Vivi- 

 parus, Cerithium, Scala, Aporrhais, Dicroloma ( = Alaria), Avellana. 



