280 MINERALS AND GEOLOGY 



snails, are terrestrial ; others, as the limnea, and planorbis, 

 species of which are of common occurrence in our lakes, and 

 streams, inhabit fresh-water ; but the greater number are marine 

 types. 



The class may be subdivided into four sub-classes : Pulmonata, 

 Opisthobranchiata, Placophora, and Prosobranchiala the great ma- 

 jority of species belonging to the latter. 



Pulmonata : The animals of this sub-class possess, in place of 

 branchiae, a simple lung-sac by which they breathe air directly from 

 the atmosphere. They comprise land and fresh-water types, and are 

 chiefly represented by the genera Helix, Planorbis, Limnea, &c., fos- 

 silized examples of which, belonging to recent species, occur in some 

 of our Post-Glacial deposits ; but the sub-class dates from at least the 

 Carboniferous period. All the Pulmonata are hemaphrodites. 



Opisthobranchiata : This sub-class comprises marine types in 

 which the branchiae are situated behind the heart. Like the Pulmon- 

 ata (and the preceding class of Pteropods) all are hemaphrodites. 

 They are commonly divided into Nudibranchiata (e. g. Eolis, Doris] 

 in which there is no shell ; and Tectibranchiata (e. g. Bulla, Actceo- 

 nella, &c. ) mostly with external shell. The latter date from the Tri- 

 assic period, but are of no special interest. 



Placophora : in this sub-class there is only one family, the Chi- 

 tonidce, dating from the Silurian period but without known repre- 

 sentatives in Canadian strata. The Chitons are abnormal gastero- 

 pods of flat, oval or elongated form, in which the upper surface of 

 the body is protected by a series of eight more or less tile-like plates. 

 They are marine, sexually-distinct types. 



Prosobranchiata : This sub-class includes all the typical braiichi- 

 ferous gasterpods in which the branchiae are placed in front of the 

 heart. The species are marine forms with distinct sexes. The shell 

 is commonly more or less spiral ; but whilst in some forms the spire 

 is very long, in others it is quite short or almost obliterated ; and in 

 some few genera the shell is patelliform. The sub-class admits of a 

 natural division into two leading groups or sections : Holostomata 

 and Siphonostomata . 



