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 ; b, 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 



L 



COMMON MUSSEL, OPENED TO SHOW THE VARIOUS 



organs (nat. size). 



DATE SHELLS IN THE ROCK (liat. size 



ruins must at some time have been submerged, and that the coast has changed its 

 level within historic times at several epochs. 



