OF CONCHOLOGY. 37 



513 the side which is sessile must be 

 considered as the base, or under part. 

 Thus in the Serpula Liicidu the fixed 

 part is sometimes very small, and the 

 mouth protends spirally upwards, in 

 a contrary direction to the sun ; and 

 therefore must be considered a re- 

 versed or heterostrophe shell, the same 

 as if the volutions nearest the mouth 

 had turned laterally upon the centre 

 or fixed ones. This shell, indeed, is 

 most frequently found with regular 

 lateral volutions ; and though subject 

 to great variety, with respect to con- 

 tortions, it invariably turns the aper- 

 ture one way. 



" In some species of Nautilus,, bow- 

 ever, there can be no rule to ascertain 

 whether it is dextral or sinistral ; for 

 when the aperture is exactly lateral, 

 the lip collapses the body equally, and 

 the sides of the shell similar, as in the 

 Natilus Calcar, it cannot be defined. 

 Jn others of that genus, as in N, Bee- 



