CLASSIFICATION 19 
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, Tornatina, 
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 VL, 
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” or “continuous” ; 
