162 MOLLUSCA. GASTEROPODA. 



ORDER. OPISTHOBRANCHIATA. 



The gills and auricle are placed behind the ventricle. 

 A shell is absent in some forms. All are hermaphrodite 

 and all marine. 



Bulla. Shell solid, smooth, sub-globular or ovoid, 

 convolute. Spire concave. Aperture as long as the last 

 whorl, rounded at the ends, widest in front ; outer lip 

 sharp. Cretaceous to present day. 



ORDER. HETEROPODA. 



The forms included in this order live in the open ocean 

 at the surface. The foot is modified so as to form a 

 vertical fin ; a shell is absent in some and when present is 

 always thin. Only a very few forms occur fossil. 



ORDER. PULMONATA. 



Respiration takes place by means of a pulmonary 

 chamber. A shell may be present or absent, and is 

 holostomatous. All the genera are hermaphrodite, and 

 nearly all live on land or in fresh water. 



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. 

 Purbeck Beds to present day. Lives in fresh water. 



Flanorbis. Shell discoidal, horny, whorls numerous. 

 Aperture oblique ; peristome simple, sharp. Jurassic to 

 present day. Freshwater. 



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

 perforate or not ; aperture oblique. Peristome simple or 

 reflected. Eocene to present day. Lives on land. 



