120 



A POPULAR EXPOSITION OF THE 



number secrete an external and univalve shell, but some few, as the 

 common slug, are " naked " or possess merely a rudimentary shell; 

 and in the chitons the shell is composed of several pieces. Some 

 gasteropods, as the common snail, are terrestrial. Others, as the 

 litnnea, paludina, and planorhis, species of which are so common in 

 our lakes and streams, inhabit fresh-water ; but the greater number 

 inhabit the sea. The class may be subdivided naturally into two 

 leading groups : Branchifera or water-breathers, and Pulmonifera or 

 air-breathers. 



The Branchifera, furnished with gills or branchiae for breathing the 

 air contained in water, include all the fluviatile and marine types. 

 They fall into two sections : Siphonostomata 

 and Solostomuta. In the former, the opening 

 or so-called " mouth " of the shell is more or 

 less deeply notched at one or both extremities, 

 OP is otherwise lengthened into a kind of slit 

 tube or " canal." The species are marine, and 

 all are carnivorous. Comparatively few occur 

 in the lower fossiliferous rocks, the place of 

 the carnivorous gasteropods having been 

 apparently supplied in great part, in the early 

 geological epochs, by numerous predatory 

 cephalopods. An example of this section is 

 shewn in fig. 120, representing a species of Buccinum (closely allied 

 to the existing B. undatwm, if not identical with that species,) irom 

 the Post- Tertiary deposits of Eastern Canada. 



In the Holostomata, the aperture of the shell has an uninterrupted 

 and more or less circular margin. The species are almost entirely 

 vegetable-feeders. Representatives occur in all the fossiliferous 

 rocks, and are numerous in existing Nature. The annexed figures 



Fig. 120. 



fig. 121. 



Fig. 12a. 



