378 THE MEDALS OF CREATION. Cuap. XI. 
towards the observer is in the same relative position as the 
person looking at it ; viz. the HEAD in front, and the right 
and left valves in their natural relations. The length of the 
shell is therefore from the front to the back of the animal : 
the width or transverse diameter is from the umbo to the 
margin. Much confusion has arisen from many concholo- 
gists having described the length and width of a shell 
diametrically opposite to the proper position of its inhabi- 
tant. 
II. The EncepHaLous Moriusca.—These possess a head, 
with feelers or soft tentacula, eyes, and a mouth with jaws ; 
they are arranged in classes, according to the modification 
of their locomotive organs ; for, with but few exceptions, 
they are free animals, and can crawl, climb, or swim. Their 
shells are, for the most part, composed of one piece, or valve, 
hence they are termed Univalves. In some genera the shell 
is a simple cavity, spirally disposed, as in the Snail; in 
others, it is conical, consisting of one or many pieces, as in 
the Limpet and Chiton. In the Cephalopoda it is internally 
divided’ into cells, or chambers, as, for example, in the 
Nautilus. 
The Encephalous Mollusca are subdivided into the fol- 
lowing classes ; viz.— 
a. Prrropopa (wing-feet).—In these the organs of pro- 
gression are two wing-like muscular expansions, proceeding 
from the sides of the neck, by which they can swim and 
float in the open sea: all the species are of small size. 
b. GASTEROPODA (feet wnder the body).—These crawl by 
means of a muscular disk,or foot, which is attached to the 
under-part of the body ; most of the species are marine, but 
some are terrestrial, and others inhabit fresh-water. They 
are very widely distributed ; the garden snail is a familiar 
instance of a terrestrial Gasteropod. 
c, CepHALOPoDA (feet around the head).—The mollusea of 
