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 Neritae; 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 Patella 
signifies, assume the shape of various little dishes, 
affixed by their tenants firmly to the rocks, with 
the apex uppermost. 
