310 University of California Publications in Zoology [Vol. 19 



in being possessed of the characteristic habit of boring, resuming this 

 habit in the first favorable material offered even in the quiet of a 

 laboratory aquarium, unprovoked by such conditions of its natural 

 environment as might be supposed to have caused the boring habit. 



SUMMARY 



An isopod, Sphaeroma pentodon Richardson, occurring abundantly 

 in marshes along the margin of San Francisco Bay, California, is 

 found in one locality near Rodeo, on the shore of this bay, boring into 

 ledges of soft San Pablo sandstone and Pinole tuff. Specimens of this 

 species, kept in laboratory aquaria, excavated characteristic holes in 

 blocks of chalk and were observed to use their mandibles in biting 

 away particles of the chalk, which were washed out of the bore by the 

 action of the swimmerets. The habit of boring was probably devel- 

 oped in this genus and in the related wood-boring genus, Limnoria, 

 from attempts to ream out crevices in which most of the genera of the 

 family Sphaeromidae habitually seek protection, becoming a perma- 

 nent characteristic in Limnoria and a less extensive habit in a few 

 species of Sphaeroma. 



BIBLIOGRAPHY 



Bate, C. S. 



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 Hedley, Charles 



1900. The marine wood borers of Australia and their work. Rep. Australian 

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