406 
MOLLUSCS. 
the world; one very large species from the shores of California sometimes reaching 
fully 9 inches in length. They anchor themselves by a byssus, but also have the 
power of moving from place to place, by 
casting off the byssus, extending the foot 
in the direction they determine to proceed, 
and attaching a byssal thread, which 
supports the animal while the foot is 
again extended and another thread spun. 
This process is repeated again and again, 
and thus progress is made. The structure 
of the animal may be understood by 
observing the accompanying illustration, 
where a represents the edge of the 
mantle; 6, the foot; c, the byssus; d.e, 
the foot-muscles; /, the mouth; g, the 
labial palpi; h, mantle-lobe; and i.j, the 
inner and outer gill-plates. To the same 
family belong the date-shells ( Litho - 
domus), which, as shown in the accom¬ 
panying illustration, are stone - borers. 
The shells are date-like, thin, and covered 
with a yellowish or brown periostracum. 
The boring is chiefly effected by the foot. 
The columns of the temple of Serapis at Puteoli are perforated by a species of 
these molluscs, at a point far above the present sea-level, thus showing that these 
COMMON MUSSEL, OPENED TO SHOW THE VARIOUS 
organs (nat. size). 
DATE SHELLS IN THE ROCK (nat. size). 
ruins must at some time have been submerged, and that the coast has changed its 
level within historic times at several epochs. 
