BIVALVE MOLLUSCANS. 237 



parts. Body commonly defended by a calcareous 

 shell, to which it adheres only by one or two 

 points, but in some instances it is externally 

 naked, and has an internal bone. 



The Molluscans may be .divided into several 

 families, and those of Cuvier are mostly natural, 

 but as my plan has been to ascend from the 

 lowest grade of animals towards the highest, I 

 shall reverse this order, and begin my observa- 

 tions with the last of his families, or more pro- 

 perly speaking Orders, excluding for the present 

 the Cirripedes of Lamarck, or most of the multi- 

 valves of Linne, as leading off laterally towards 

 the Crustaceans. 



His first order he calls Acephales, or headless 

 Molluscans, it includes all the bivalve shells of 

 Linne, with the addition of the Pholads or 

 stone-borers. 1 Lamarck has divided it into two 

 sections, which, regarding it as a Class, are with 

 him Orders ; the first is Bimuscular? having two 

 attaching muscles, and two muscular impres- 

 sions ; and the second is Unimuscular, 3 having 

 only one such muscle with one impression. With 

 regard to their habits and economy, the bivalve 

 Molluscans may also be divided into two sec- 

 tions, the first of which may consist of those 

 that inclose themselves either in a cell or burrow, 

 or live in the mud, &c. ; and the second of those 



1 Pholas. 2 Conchifires dimyaires. 



3 C. monomyaires. 



