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scription of the genus, and some interesting and expla- 

 natory facts respecting the structure and growth of the 

 shell ; which will satisfactorily account for a singular oc- 

 currence, frequently observed, of the young and incom- 

 pletely formed shell being often twice as large as the 

 adult of the same species. In the full grown and mature 

 «hell, the form is oval, convex above, slightly flatten- 

 ed beneath, and the spire nearly covered by the evo- 

 lutions of the whorls, leaving in some species a small 

 cavity, resembling an umbilicus ; the aperture narrow, 

 extending the whole length of the shell, and dentated 

 or plaited on either side; the right margin always re- 

 flected inwards. But in young and immature shells, 

 the form is very different, the aperture then being much 

 wider, particularly at the base; the right margin sharp, 

 and not reflected; no plaits on the columella, the spire 

 very much produced, and the shell more resembling a 

 cone, except in thickness. Tn the next stage of growth, 

 it acquires the outline of its perfect form, but it is not yet 

 completed, as its substance is thin, the shell light, and 

 the spire, though very small, not so much concealed as 

 inthe succeeding stage of increased growth: thepattem 

 and coloring -are also very different to those of the third 

 and last stage, at which the shell becomes solid; and 



