CLASSIFICATION. 
O 
combined into classes. All these divisions should possess in com- 
mon, some peculiarity of greater or lesser importance, and all these 
groups are or may be sub-divided into section.s, which with the prefix 
of Sub-, indicate the pos.session of characters of lesser importance 
than those distinguishing the chief groups. 
The Animal Kingdom is divideil into several Sub-kingdoms, the 
Vertebrata standing at the head, and by general concurrence among 
scientists, the Mollusca occupy the second place, preceding the Arti- 
cnlata and every other sub-kingdom. 
The ^lollusca may be primarily divided into four Classes, based 
upon the modifications of the foot or locomotive organ, and named 
Cephalopoda, Gastropoda, Sca^hopoda, and Pelecypoda. The 
Pteropoda, which formerly constituted a fifth class, are now con- 
sidered to be Opisthobranchiate Gastropods. Two of these classes 
are exclusively marine, leaving only the Gastropoda and Pelecypoda, 
to which groups our British land and freshwater shells exclusively 
belong. The names Gastropoda and Pelecypoda, refer to the 
morphological peculiarities of the animals, the alternative terms 
Univalve and Bivrdve, which are also in general use, respectively 
expressing the character of the shells. 
The Gastropoda are characterized by the development on the 
ventral side of the body of a sole-like locomotive disc or foot, and by 
Fig. 1. — An Anisopleurous Pulmonate Gastropod. 
Helix aspersa v. zonata Moq., 
Folkestone, collected by Mrs. Fitzgerald. 
its uudulatory or wave-like expansions and contractions the creature 
moves. This class may be primarily divided into two groTips or 
Sub-classes, Isopleura and Anisopleura, according as the chief 
viscera have or have not been subjected to a toi’sion and a semi- 
rotation, bringing the termination of the alimentary canal from its 
presumed ancestrally posterior and medial position, towards an 
anterior or lateral one, involving in this movement other organs and 
their ducts ; this change of position is assumed to have been caused 
