26 



POPULAR CONCHOLOGY. 



Mollusca are divided into two great classes, 

 Univalves and Bivalves : the first comprising all 

 those whose shells are formed of one single 

 valve or piece, as the snail, and the latter, those 

 which have shells consisting of two valves, or 

 distinct pieces, as the oyster. Each of these 

 classes is again divided into orders, each order 

 into families, each family into genera, and each 

 genus into species. These will be treated on in 

 succession, according to a systematic arrange- 

 ment, extending as far as the different genera : 

 the limits of this work, which is intended but as 

 an introduction to the science, will not allow 

 of a description of each species, and a mere dry 

 list of their names would be uninteresting. 

 Reference can easily be made by the reader, 

 who has mastered the subject as far as it will 

 be pursued here, to the many large and scien- 

 tific works which have appeared, in which each 

 specific variety is introduced : that they would 

 extend these remarks much beyond a short 

 treatise, is evident from the circumstance that 

 one genus alone, the Helix, or snail, comprises 

 nearly 250 different species. 



The classes Annulata and Cirrhipeda form 



