Outver.—Marine Littoral Plant and Animal Communities. 505 
(3.) The shell takes the form of a cone spirally twisted and fused into 
a fusiform or turbinoid shape, with the last turn or body-whorl usually 
capable of being closed by an operculum. The animal can withdraw itself 
partly bury themselves when the tide is out. Alcithoe arabica, Cominella 
adspersa, and Ancilla australis have large open canals, and frequent sand- 
flats, where they creep along just beneath the surface, thus keeping the 
air. 
P .—In the Pel da the shell consists of two valves laterally 
elecy poda e Pelecypo equ e apum 
(Modiolus, Mytilus, Ostrea) ; the valves fit closely along their margin except, 
in the Mytilidae, at one point on the ventral side; but here the organ of 
shells, and these live at or below low-tide mark. Thus the perfection 
f the shell fit bears a relation to the position 
of the animal in the intertidal belt, and, as with. gasteropods, though the 
pelecypod shell is primarily a weapon of defence, it is essential as a conserver 
of moisture to those animals regularly left dry by the tide. 
ee i sed of eight overlapping shelly valves, 
eura.—The shell is compo ght o Er NR 
