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, 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 cireumstances connect- 
ed with the characters of the Shell. 
The arrangement of the testaceous mollusca, according 
