CYCLOSTOMA. 



347 



species from the coast and islands of Eastern Africa, are 

 generally solid, heliciform shells, with the aperture decid- 

 edly angular posteriorly, and with a bony operculuin of 

 only two or three spirals. The South American spe- 

 cies are planorboid, with a broad umbilicus, a perfectly 

 circular aperture, and a horny, multispiral operculum. 

 In the West Indies is an extensive group of a pupoid 

 form, cancellate surface, truncated tip, circular aper- 

 ture and calcareous, multispiral operculum, with the 

 edges of the spiral projecting so that it appears lamin- 

 ated. In the Pacific, we have one or two groups quite 

 different from any thing found elsewhere. Without pur- 

 suing the subject further, however, we will merely say, 

 that we believe that when a thorough comparison of the 

 species shall be made, a remarkable limitation of species 

 of a certain structure of the shell to a certain geographi- 

 cal area will be found, and also a corresponding differ- 

 ence in the structure of the animal ; consequently a sub- 

 division of the genus Cyclostoma, as it is now received, 

 will follow. G. 



