O 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 ; con- 

 stituting however, in reality, as solid and insepa- 

 rable a shell as any in which this construction is 

 not visible. Among the univalves, those which 

 are the most deceptive are theCypraia, 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 incomplete 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 expanded, 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 narrower, and the spire smaller; they 

 then assume the character of a Bulla, but still they 

 are thin ; at length they gain thickness, first one 



