PATELLA. 119 



with these two. The gradations in the scale of 

 Nature are in general so regular and yet so small, 

 that it becomes no easy task to trace the separa- 

 ting line. Class is linked to class by an order 

 trespassing on both ; order to order by an interme- 

 diate genus ; a doubtful species unites two genera ; 

 and varieties confound the limits of a species. 

 This curious course is well illustrated by the ob- 

 servations we may make on the genus Patella, 

 placed as it were on the boundary between those 

 shells which have a regular spire and those which 

 have it not. 



Some few of the chambered section possess a 

 slight resemblance to Neritae ; but upon close exa- 

 mination it is found that the margin rises above 

 the flattened dissepiment on all sides, which is 

 therefore not to be compared with the columellar 

 lip of the Nerita. The remaining divisions are of 

 the simplest form ; and, as their original name 

 TrareXXa signifies, assume the shape of various lit- 

 tle dishes, affixed by their tenants firmly to the 

 rocks, with the apex uppermost. 



