4 FIRST BOOK OF ZOOLOGY. 



In some shells the spire is elongated; in others the spire 

 is short; in others still the spire is dej>ressed or flattened. 



Fio. 5. 



4. If the shell is held in the hand, with the aperture 

 toward the holder, and the spire pointing upward, as in the 

 figures drawn, the aperture will be either toward the right 

 hand, or toward the left hand. In the figures already given, 

 the aperture is on the right hand, and these shells are called 

 dextral, or right-handed, shells. 



Shells having the aperture on the left hand when held in 

 the way above described, are called sinistral, or left-handed, 

 shells. Let the pupils here examine all the shells they have 

 collected, holding each one with the spire pointing upward, 

 and the aperture toward them, and sepa-rate the dextral 

 shells from the sinistral shells. As sinistral shells are not 

 as common as the other kind, it may be that none will be 

 found in the first collections made by the pupils. The fol- 

 lowing is a figure of a sinistral shell : 



Fig. 6. — Sinistral Shell. 



