8 
INTRODUCTION. 
ible by new external layers, extending each beyond 
the margin of its predecessor ; and in others the 
valves appear to be formed of a gradation of suc- 
cessive sizes laid one under the other, united only 
at the beak, and capable of being separated ; consti- 
tuting however, in reality, as solid and inseparable 
a shell as any in which this construction is not vis- 
ible. Among the univalves, those which are the most * 
deceptive are the Cyprsea, Buccinum, Strombus, 
and Murex genera. The young of the others are 
chiefly distinguishable by the unfinished edge of 
the outer lip : this is either thin, notched, or incom- 
plete in its dimensions ; still there is no difference 
so great as materially to mislead. Some of the 
Cypreese in their young state exhibit the columella 
much plaited,^ the aperture broad, the outer lip ex- 
panded, and the spire considerably raised above 
the body; they now resemble species of Voluta. In 
the progressive stages the columella becomes less 
plaited till it be quite smooth, the aperture nar- 
rower, and the spire smaller; they then assume 
the character of a Bulla, but still they are thin ; at 
length they gain thickness ; first one lip and then 
