164 MOLLUSCA. GASTEROPODA. 



also found on land. Some marine genera, such as Littorina, 

 Cerithium, and Purpura, are able to live in fresh as well 

 as in salt water ; on the other hand some freshwater forms 

 are at times found living in the sea, e.g. Limncea, Neritina, 

 Bithynia, and Planorbis; this is especially the case in 

 places where the water is less salt than the main mass of 

 the ocean, as for instance in the Baltic, where we find the 

 genera just mentioned living side by side with Littorina 

 and with the marine lamellibranchs Cardium, Tellina, and 

 Mya. Of the six orders of the Gasteropoda, four are 

 entirely marine, viz. the Polyplacophora, Heteropoda, 

 Opisthobranchiata, and Pteropoda; arid the Prosobranchiata 

 are mainly marine. The Pulmonata are found only on 

 land and in fresh water. The Heteropoda and Pteropoda 

 live in mid-ocean at or near the surface, whereas the other 

 marine forms occur mainly at no very great distance from 

 the coasts and in comparatively shallow water. 



In connection with the Gasteropoda a few words may 

 be said with regard to the distribution of the marine 

 Mollusca generally. Just as we find on land, that the 

 surface of the earth can be divided into regions, each 

 characterised by the presence of certain animals riot found 

 elsewhere, so also in the oceans we have similar life- 

 provinces, caused mainly by conditions of climate and 

 food supply. These are marked by the abundance of 

 certain genera and the presence of some species which do 

 not extend into other provinces ; but they are less distinct 

 than the terrestrial regions, since very few of the genera 

 are confined to one province. In the European region the 

 chief provinces are : the Arctic, which includes the polar 

 seas and extends as far south as the North of Iceland and 

 the North Cape ; the Boreal, extending from the last 

 down to near the southern end of Norway and including 



