250 MOLLUSCA. GASTEROPODA 



Fossil Pteropods occur in Upper Cretaceous and 

 Tertiary formations and belong mainly to genera which 

 have living representatives. In the Silurian and Devonian 

 rocks large numbers of small conical smooth shells, which 

 resemble the living Styliola, are found, and may possibly 

 be the remains of Pteropods. 



SUB-ORDER II. PULMONATA 



The mantle-cavity is modified to form a lung. There 

 is no gill. An operculum is nearly always absent in the 

 adult. The Pulmonata are mainly land and fresh-water 

 forms. 



Limnaea. Shell spiral, thin, horny ; last whorl very large, 

 spire sharp. Aperture large, oval, rounded in front. Columella 

 more or less twisted. Peristome sharp, entire. Pur beck Beds to 

 present day. Lives in fresh water. Ex. L. stagnalis, Pliocene to 

 present day ; L. longiscata, Headon Beds, etc. 



Planorbis. Shell discoidal, horny, whorls numerous. Aper- 

 ture oblique ; peristome simple, sharp. Jurassic to present day. 

 Fresh water. Ex. P. corneas, Red Crag to present day ; P. eitom- 

 pkalus, Headon Beds. 



Helix. Shell variable — conical, discoidal, or globular ; with or 

 without an umbilicus ; aperture oblique. Peristome simple or re- 

 flected. Eocene to present day. Lives on land. Ex. H. nemoralis, 

 Pleistocene and living. There are numerous sub-genera. 



Distribution of the Gasteropoda 



Some of the Gasteropoda live on land, others in fresh 

 water, but the majority are marine ; they are found in the 

 seas of all parts of the world but are especially abundant 

 in warm regions and in comparatively shallow water. A 

 few forms can exist both on land and in water, e.g. 



