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 Cypraea, 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 

 Cypraeae 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 



