PATELLA. 107 



rating 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. At least we may boast of somewhat 

 more accurate ideas upon the subject than those 

 naturalists possessed, who, about two centuries 

 ago, placed Patellae among the Bivalves, because 

 the stone to which the animal adhered served for 

 a second valve. 



Some few of the chambered section possess a 

 slight resemblance to Nerita} ; but upon close ex- 

 amination, 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 name Patellae 

 signifies, assume the shape of various little dishes, 

 affixed by their tenants firmly to the rocks, with 

 the apex uppermost. 



