198 HEUCID^E. 



their axis in the same plane, serve to distinguish it from 

 other species. It varies in being occasionally a little 

 convex, more or less carinate, and in exhibiting a greater 

 or less number of full volutions on the base. 



The reflected lip in this shell seems to be formed at 

 various periods of growth, thus creating a greater diver- 

 sity of size in the apparently mature shell than exists in 

 any other species. From the nucleus until the accom- 

 plishment of five full whorls, each whorl on the base is 

 curved a little lower than that which precedes it ; and 

 up to this time, consequently, the umbilicus is deep and 

 gradually expanding, exhibiting, when carefully exa- 

 mined, all the volutions. Up to this period, also, the 

 spire is almost always prominent. After five whorls are 

 completed, the succeeding ones usually follow in the 

 same horizontal plane, and give a discoiclal character to 

 the shell. It is manifest, therefore, that specimens in 

 each of these stages must present considerable differ- 

 ences ; and, accordingly, the small, delicate shell, having 

 a slightly convex spire of five whorls, a deep umbilicus, 

 and a transverse diameter of only one-eighth of an inch, 

 forms a beautiful variety, and has been thought to be a 

 distinct species. 



The name planorbula was applied by Lamarck, in 

 defiance of received rules of nomenclature, as his refer- 

 ences show that he knew that another specific name was 

 already in use. It seems probable that this shell was 

 first described by Megerle, in the Berlin Magazine, 

 1816, as H. cereolus ; but as both his description and 



