i io PALEONTOLOGY 



falls naturally into two divisions, the Opisthobranchiata, 

 marine forms with gills, and the Pulmonata, terrestrial 

 and freshwater forms in which the mantle-chamber is 

 converted into a lung-sac. These two latter groups are 

 so distinct that many systematists raise them to the rank 

 of sub-classes, and as this is convenient for palaeonto- 

 logical purposes we will do so here. No shell character 

 can be pointed to as distinctive of either of these three 

 sub-classes ; nevertheless, any one who is familiar with 

 gastropod shells generally would feel little hesitation in 

 placing a genus he had never seen before in its proper 

 sub-class. He would not find it easy to state his reasons, 

 as they would consist in combinations of characters, 

 positive and negative, which could only be stated with 

 many qualifications. Among other general facts that he 

 would have in mind would be the much larger proportion 

 of fossil genera belonging to the Streptoneura than to 

 the Euthyneura (for in the latter, and especially in the 

 opisthobranchs, there is a tendency towards the reduction 

 and final disappearance of the shell) ; and the restriction 

 to freshwater deposits of Pulmonata and a few genera of 

 Streptoneura. 



SUB-CLASS: STREPTONEURA. 

 ORDER: ASPIDOBRANCHA. ' 



SUB-ORDER : Docoglossa. This includes the limpets, 

 forms with shells like Emarginula, but without the notch. 

 Forms undistinguishable from the common modern 

 limpet, Patella, date from the Silurian, and allied recent 

 genera are also known as fossils. 



