107 



that it becomes no easy task to trace the sepa- 

 rating Hne. 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 observations 

 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 accu- 

 rate 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 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 name Patella: 

 signifies, assume the shape of various little dishes, 

 affixed by their tenants firmly to the rocks, with 

 the apex uppermost. 



