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cork-fcrew, and we fliall find, that whether the mouth, 

 or apex, is placed upwards, the fpires will turn from the 

 upper to the lower end like a common fcrew, which is 

 \\\ the fame direftion as the index of a time-piece, and 

 what is commonly underftood by a dextral or right- 

 handed fcrevv^. As a further definition, fuch fhells have 

 their aperture on the right fide, when examined with 

 that end downward; and is in the diredion of the fun's 

 apparent motion. 



On the contrary, a reverfed (hell, when placed in a 

 perpendicular pofition, has its fpiral volutions in an op- 

 pofite direftion to the motion of the index of a watch, 

 or a clock, or to a common fcrew ; and in fatl refem- 

 bles what is ufualiy termed a finiflral, or left-handed 

 fcrew. Thefe have their aperture on the left fide, or 

 oppofite the left hand of the perfon holding the fiiell 

 with the mouth downward; and the opening is oppofite 

 the fun's apparent motion. 



In order to determine whether a flat fiiell, whofe vo- 

 lutions are laterally placed, is a reverfed fpecies, we 

 have only to examine which way the volutions turn from 

 the apex or centre, towards the mouth ; and if we find 

 it contrary to the motion of the index of a watch, it is a 

 heterojlrophe, or reverfed fliell : and vice verfa. 



In fome of the more deprefied fpecies of Helix, or 

 Nautilus, attention is requifite to be paid to the mouth, 



Z z in 



