48 



POPULAR CONCHOLOGY. 



they vary in size, from a few inches to two feet in length ; 

 they are found reversed or sinistral, as well as with the 

 spire in the usual direction. 



Helicoceras. If Orb. — Shell many-chambered, spiral, 

 rolled up either to the right or left, but 

 the whorls do not touch, being com- 

 pletely separated from each other: the 

 mouth oval and entire ; the partitions 

 formed like those of Turrilites, the si- 

 phuncle lying above. — 11 species, fossil. seutoceras rotunavm. 



Class II. — GASTEROPODA. (Cuvier.) 



The class Gasteropoda forms the most numerous division 

 of the Mollusca. The animals move regularly by means 

 of a fleshy disk or foot, using it either for creeping or 

 swimming. On the back is a mantle, more or less dilated, 

 which in most species produces a shell, though in some 

 it is merely rudimental, and in a few instances entirely 

 absent. The head is always clearly distinguishable, and 

 bears from two to six feelers ; these are rarely wanting. 

 The eyes are generally small (largest and most perfect in 

 the Strombus), and are situated sometimes directly on the 

 head, sometimes on separate stalks, or at the bottom, 

 middle, or extremity of the feelers. The lungs display 

 considerable diversity : some Gasteropoda breathe the air 

 at once into apertures, the walls of winch are lined with a 

 vascular network (as land snails) ; others inhale air mixed 

 with water through gills (as most sea molluscs) ; others, 

 again, seem to possess no particular organ of respiration. 



