xi PHYLUM MOLLUSCA 287 



/.., if we begin, at the apex of the spiral, to reach the open- 

 ing of the shell we have to pass from left to right with the 

 columella always on our right-hand side : in a few cases, 

 however, the spiral is sinisfral, taking the opposite direction 

 from that of the ordinary dextral shell. The form of the 

 shell varies with the degree of obliquity with which the 

 whorls are set on the axis. When the obliquity is very 

 slight (Fig. 162) the spiral is nearly flat ; when the obliquity 



FIG. 162. Shell of Solarium perspectivum from the under side. 

 (From the Cambridge Natural History, ,) 



is great, an elongated tapering shell, such as that repre- 

 sented in Fig. 163, is the result. Sometimes the later whorls 

 completely cover over the earlier ones, so that the spiral 

 form of the shell is concealed. In some cases only the 

 apical portion is spiral, the remainder being a straight or 

 sinuous cylinder. The mouth of the shell has usually a 

 prominent margin or peristome, which is sometimes entire 

 and continuous, sometimes broken by a deep notch or 

 a spout-like prolongation or canal (Fig. 160), formed in 

 connection with the development of a spout-like pro- 

 longation of the mantle, the siphon, which lies in it. The 

 mouth of the shell in many Gastropoda is capable of 



