MOLLUSC A. GASTEROPODA 251 



Ampullaria, which commonly lives in lakes and rivers, 

 and is 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 

 fresh -water 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 lamelli- 

 branchs Cardium, Tellina, and Mya. Two divisions of the 

 Gasteropoda are entirely marine, viz. the Isopleura and 

 Opisthobranchia ; the Sbreptoneura (or Prosobranchia) are 

 mainly marine. Nearly all the Pulmonata are found on 

 land or in fresh water. 



In connexion with the Gasteropoda a few words may 

 be said with regard to the distribution of the marine 

 Mollusca generally. Some forms live in mid-ocean at 

 or near the surface ; these include the Pteropods, the 

 Heteropods and a few other Gasteropods, as well as many 

 Dibranchiate Cephalopods. Others live on the sea-bottom 

 and are found at almost all depths, but are much more 

 abundant in shallow than in deep water. The deep-sea 

 species have a wide geographical range, owing to the 

 uniformity of the conditions under which they live. The 

 Mollusca which occur in moderate depths have a smaller 

 range and are distributed in regions or provinces, limited 

 mainly by conditions of climate. Each province is marked 

 by the abundance of certain genera and by the presence of 

 some species which do not extend into other provinces; 

 but they are not separated by any sharp line, and but few 

 genera are confined to any one province. In the European 



