SECTION VI. 125 



as thin as river Shells, e, g. the Paper 

 Nautili, Partridges, &c. Mr. Davila has on 

 this error placed the fmall blue rayed, 

 and the cracked Limpets, as river Shells, 

 which are fea Shells, 



' In regard to land Snails, many of them 

 are as thick and well coloured as any fea 

 Shells ; therefore this charadler is alfo very 

 vague and uncertain. 



However, one character is very fixed, or 

 a ftandard in the land operculated Uni- 

 valves, and is extremely different from the 

 fea operculated Shells. I took notice of it 

 in my account of the opercula or lids. It 

 is, the opercula, or lids, of all Land Shells, 

 are never wrought with fpiral lines, are of a 

 tough or leathery fubftance, and not ftoney 

 or cartilaginous; and befides, they are never 

 attached to, or make any part of the very 

 animal. It is extremely feldom that we can 

 elucidate this particular in colle£lions ; as 

 Shells and their opercula are rarely feen to- 

 gether. 



