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CERITHIUM. 



and if separated would compose the Genus Potamis if 

 expressed in Latin. They appear to be nearly related to 

 Melania and Melanopsis or Pireiia. Ihese being ex- 

 cluded, we have reason to consider Lamarck's Cerithmm 

 as a perfectly natural Genus. The species arc apparently 

 related on one side to Planaxis, and on another to Tereba, 

 from the former they may be distinguished by their canal 

 (which Planaxis has not) and from the latter by the form 

 of the columella and the reflection of the canal. From 

 Pirena the Ceriihia may be easily distinguished by the 

 posterior part of their outer lip being entire. The Centhia 

 are usually much lengthened or turrited spiral univalves, 

 consisting for the most part of numerous volutions, 

 generally more or less tubercular or spinose or rough 

 outside, seldom nearly smooth or spirally grooved. The 

 aperture is subquadrate or elliptical, with a somewhat 

 thickened peritreme, which is sometimes broadly rejected; 

 upper part of the aperture modified within by the last 

 volution ; columella arched, with an acute spiral told at 

 its base forming the upper edge of its rather short, 



frequently reflected canal. . , , ;i 



Numerous indeed are the species, both recent and 

 fossil, which may properly be placed in this Genus, although 

 very few of them belong either to our own or the European 

 shores There is however so much variation m the 

 external rugosities of the shells that it is exti;emely 

 difficult to ascertain what are to be considered as distinct 

 species and what are merely varieties. There are several 

 peculiarities of form to be observed among the Centhia ; 

 we shall notice the most important. 



1 With a very short spire, granular surface, nearly 

 circular aperture and exceedingly short, not projecting 



''^"^2 plcilated at its full growth and haying a much 

 dilated and reflected outer lip, ribbed longitudinally on 

 the surface, with scarcely any canal, and a very circular 

 aperture^ Jig^ 2. ^^^^ ^.^^^^^ reflected outer lip 

 continuing over the canal at the anterior extremity and 

 S g i^^felf to the end of the columella, so that the canal, 

 wMch is not reflected forms a small, sharp- edged, circular 

 aperture- these are ribbed externally and decussated. Fig.3. 



