CLASSIFICATION ig 



exserted towards the left side (Plate IV., Fig. 3, 

 B', C')« In the first case the shell will be dextral, 

 in the latter sinistral. With sinistral animals these 

 conditions are just reversed. 



In certain forms (Odostomia, Turbonilla, Tomatina, 

 etc.) the young shell is sinistral, after which dextral 

 growth suddenly ensues, and the completed shell is 

 dextral (Plate IV., Fig. 2). The term "hetero- 

 strophy " has been applied to this condition. 



The axis, or " columella," of the shell is sometimes 

 hollow, or " umbilicated " (the hollow itself being 

 called the " umbilicus " (Plate VII., Fig. 9 ; and VI., 

 Fig- 3) > sometimes the whorls are closely coiled, and 

 a solid pillar of shell results (Plate VI., Fig. 2). In 

 the genus Natica the umbilicus is open, but in many 

 of the species the cavity is more or less filled by a 

 shelly deposit known as " callus " (Plate VI., Fig. 3, 

 A-E). The "apex" or extreme top of the shell 

 generally differs in markings and other features from 

 the rest, and offers important characters. This 

 " nucleus," or " protoconch," is the portion formed 

 in the egg } hence it is also known as the " embryonic 

 shell." 



The spiral channel formed by the junction of the 

 whorls is termed the " suture." A spiral line traced 

 along the whorls midway between the sutures would 

 mark the " periphery." The " mouth," or " aperture," 

 has sometimes quite a circular margin (peristome), 

 when it is said to be " entire 



