26 MOLLUSCA. 



CHAPTER II. 



PROGRESS OF THE SCIENCE. 



NATURALISTS have pursued a variety of methods in their 

 examination of this important branch of Zoology, and have 

 proposed systems of arrangement founded on very different 

 principles, and marking different epochs in the science. In 

 the methodical distribution observed by some, the form of 

 the shelly covering has been exclusively attended to, while 

 the organization of the animal itself has been overlooked, or 

 even disregarded. A few have made the habits of the ani- 

 mal, the groundwork of their system. Others have passed 

 over the characters exhibited by the forms and structure of 

 the shell, and have confined their attention exclusively to 

 the form and structure of the contained animal. Lastly, 

 there have been a few, who, embracing all the circumstan- 

 ces connected with the shell r the animal, and its habits, have 

 constructed systems at once natural and convenient. In the 

 following sections we propose to consider these four classes 

 into which the cultivators of this department of science may 

 be distributed. 



SECT. I. Systems constructed from circumstances connect- 

 ed with the characters of the Shell. 



The arrangement of the testaceous mollusca, according 



