8 
GASTEROPODA. 
It is equally easy to determine the natural position 
of the bivalves without the presence of the animal ; 
for the ligament is always placed on the dorsal sur- 
face of the animal^ and the mouth is placed on that 
side of the apex of the valve, or umbo, which is be- 
fore the ligament. Consequently, if a bivalve shell 
is placed on the table, with its hinge-side uppermost, 
and with the ligament towards the observer, the shell 
will be in its natural situation, and the sides of the 
shell will agree with the sides of the observer. 
It is to be remarked that Linnseus, and the natu- 
ralists of his school, described what is here called the 
front of the shell as the back, the left valve as the 
right, and vice versa ; and Lamarck in general (but 
not universally) followed the same rule. The me- 
thod above described is, however, so obviously cor- 
rect, and every other so liable to confusion from the 
want of a sound foundation, that it cannot fail, 
sooner or later, to be universally adopted. 
Class I. GASTEROPODA. 
The adult animal furnished with a more or less dis- 
tinct foot, placed under the body, for crawling on 
the ground or the surface of the water. Having a 
distinct head furnished with eyes and tentacles, they 
are usually protected by one or by two unequal-sized 
valves, the largest being conical, spiral, and enclosing 
the greater part of the body, and the smaller forming 
