In using the clam key one should remember that the hinge is regarded 

 as the most dorsal region, and that the hinge is posterior to the beak or umbone. 

 The early American conchologists sometimes forgot the latter point, and as 

 a result their descriptions do not always agree with their figures. If the shell 

 is held so that the hinge is on top and level, it will greatly facilitate determin' 

 ing whether the posterior ridge runs to the mid-posterior or to the post-basal 

 point, often a useful character for identification. A quick method of ap 

 proximating proportions is the stunt of placing the valves together with one 

 valve across and at right angles to the other. This makes it at once apparent 

 whether the shell is twice as long, or less or more than twice as long, as its 

 height. For more exact measurements a ruler should be placed across an open 

 valve, and the straight distance taken for height or length. 



Snail shells are right- or left-handed. An easily remembered and quick 

 method of determining the direction of coiling is to hold the shell v^'ith its 

 spire up and its aperture toward the observer. If the aperture is on his right, 

 the shell is dextral; if on his left, sinistral. Or, looking down on the spire, 

 clockwise coiling is right or dextral, counter-clockwise is left or sinistral. The 

 whorls are numbered from the center outwards, the apical whorl being the 

 first, but it is usually more convenient to count them in the opposite direction, 

 starting with the body whorl directly opposite the edge of the aperture. 



The height of a snail shell is usually considered to be the vertical distance 

 from the apex to the lower edge of the aperture. The width is the greatest 

 diameter at right angles to the axis, although sometimes in heliciform shells 

 the width is considered to be the greatest oblique diameter. The aperture of 

 the shell is usually very important from the taxonomic point of view. The 

 free edge of the aperture is called the lip, the outer side being called the outer 

 lip and the part curving upwards toward the base of the columella the inner 

 lip, and may be thin-edged or reflected. The aperture is often constricted by 

 folds or protuberances called lamellae or denticles, although for convenience 

 often referred to as "teeth", and so called in the key. 



The amateur collector may be confused by young Polygyras, which may 

 not yet have acquired the reflected lip characteristic of adult specimens. Most 

 species of this group have rather dull, yellowish shells. Most of the species 

 of Family Zonitidae, which are often confused with young Polygyras, have 

 uniformly shining or semi-shining shells. The keys are necessarily based on 

 average, adult specimens. The snail key is primarily a key to genera only, 

 although a few of the common species of some of the more widely distributed 

 genera are included. State surveys and regional books should be consulted 

 for particular localities. 



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