GENERIC SYSTEM. 53 



the latter bivalves, have not been used at all in 

 the following pages, because they appear unne- 

 cessary, and are superseded by more significant and 

 appropriate names. 



The generic characters of the multivalves are 

 derived from the situation or number of the valves; 

 of the bivalves, from the hinge ; of the univalves, 

 either from the aperture, or conformation of the 

 •hell. Generic distinctions must be permanent, 

 that is, invariably existing in all the species which 

 compose the genus. 



Specific characters are taken from the colour, 

 surface, or figure, which are different in the vari- 

 ous parts, of which each shell, in every genus, is 

 constructed. 



Varieties are formed, usually, by adventitious 

 circumstances, and differ from others of the same 

 species, for the most part, only in the painting, or 

 some non-essential shape. 



Multivalves are either parasitical, as the genus 

 Lepas ; or often unattached to foreign substances, 

 as Chiton and Pholas. 



Bivalves are divided, 1st, into those whose hinge 

 is furnished with internal teeth, either not inserted 



