354 



III order to determine wliich is really the upper fide of 

 iiie fiicll, for it is on that fide the fpiral turns are to be 

 taken from the centre or apex : and in mod inRances this 

 is to be determined by the oblique direftion of the aper- 

 ture to the under part, where the lip rarely extends fo 

 far as on the upper part. Infixed fliells, fuch txsSerpula, 

 there is no difficulty, as the fide which \?, fejjile mufl be 

 confidered as the bafe, or under part. Thus in the Ser- 

 pula lucida the fixed part is fometimes very fmall, and 

 the mouth protends fpirally upwards in a contrary di- 

 re6iion to the fun; and therefore muft be confidered a 

 reverfed or heterojlrophe fliell, the fame as if the volu- 

 tions nearefl; the mouth had turned laterally upon the 

 the centre or fixed ones. 



This fiiell, indeed, is mofl frequently found with re- 

 gular lateral volutions; and though fubjeft to great va- 

 riety with refpeft to contortions, it invariably turns the 

 aperture one way. 



In fome fpecies of Nautilus, however, there can be 

 no rule to afcertain whether it is dextral or finiftral ; for 

 when the aperture is exaftly lateral, the lip collapfes 

 the body equally, and the fides of the fliell fimilar, as in 

 N. Calcar, it cannot be defined. ' 



In others of that genus, as in A'^. Beccarii and Becca. 

 rii pcrverfus, two fhells, the principal diflinftion of 

 which, is the contrary turn of their volutions, it is eafily 



deter- 



