306 University of California Publications in Zoology [Vol. 19 



rather delicate choice on the part of the borers of those beds which 

 are best suited to receive their excavations (see pi. 15, figs. 1 and 2). 



The size of the holes made in these rocks varies from bores 6 mm. 

 long and 2 mm. in diameter, containing small and presumably young 

 isopods, to bores 35 mm. long and 9 mm. in diameter, containing large 

 and presumably adult isopods. Each bore is of approximately uni- 

 form diameter throughout its length, indicating that the animals either 

 make new bores to suit their increasing growth in size or that they 

 ream out and lengthen their original bores. Though there is no 

 direct evidence against the possibility of a progressive exchange of 

 holes, such a procedure seems altogether unlikely, in view of the 

 prodigious capacity of these animals for boring into any suitable 

 ledge, the scarcity of empty isopod holes, especially small holes, the 

 general accurate fitting of the isopods to their holes, and also in view 

 of certain laboratory observations (see pi. 17, fig. 2). 



The direction of the holes is in general downward, but not always 

 vertical or even perpendicular to the surface of the rock. Most of 

 the holes are roughly parallel in direction, but they may vary widely 

 in direction (see pi. 17, fig. 1). Observations on these rock-boring 

 isopods on San Pablo Bay have found them only between the tide 

 levels, and it is not known how deep they may live in rocks of favor- 

 able texture beneath the s\irface of the bay. Regions of deposition 

 of mud or sand, even if periodically swept by currents, would, of 

 course, be unfavorable for the boring activity of Sphaeroma, and 

 since the deposition of mud is probably a more or less continuous 

 process in San Pablo Bay, except perhaps in the midchannel, it is not 

 to be expected that these isopods in this locality bore into rock much 

 below the level at which wave action prevents the deposition of silt. 



THE PROCESS OF BORING OF SPHAEROMA PENTODON 

 RICHARDSON 



The evidence for believing that these animals themselves bore 

 the holes in which they are found is supported by the following 

 observations. 



The holes were evidently made by some active purposeful agency. 

 They are not of accidental occurrence, and it is impossible to conceive 

 of them as due to wave erosion. The surface of the bore, moreover, 

 presents the appearance of freshly cut stone. These freshly cut holes 



