GASTEROPODS 343 
Murex tenuispina. 
Acmea testudinalis, from below. 
types of marine gasteropods, and their respective characteristics 
may be noted by comparing them one with another: Fasciolaria 
tulipa (Plate LX XVI), a Floridian shell; Polynices heros, a com- 
mon species found on all the sandy shores of the American At- 
lantic coast; Murex tenuispina, an Indo-Pacific species much prized 
by collectors; and Acmea testudinalis, commonly found in Maine 
and Massachusetts at low tide clinging to rocks. 
The extreme top of the shell is called the apex, and it may be 
either sharply pointed or obtuse. Each turn is called a whorl; 
the last one, and always the largest, is called the body-whorl, the 
others collettively forming the spire of the shell. When the ani- 
mal first emerges from the egg it is always provided with one or 
two whorls, which generally may be distinguished at the apex as 
smoother than the rest of the spire, and a faint line may be 
discovered where the shell began its growth after birth. These 
