972 A. L. BARROWS — FOSSIL ROCK-BORING ANIMALS 



in which the borers are found enable us to recognize certain rock-borers 

 as such when fossilized. The occurrence of fossil nestling shells in the 

 holes of borers is even better evidence of the induration of the rock at the 

 time when the borers lived than the presence of the remains of the boring 

 animals themselves. When found as fossils, therefore, both true rock- 

 borers and nestlers indicate the existence of exposed hard rocks at the 

 time when they were either covered by sediments or raised for a period 

 out of the water, and there is evidence to suggest that these rocks entered 

 by the borers were located in access to fresh ocean water, situated in the 

 case of the echinoid and mytilid borers either at the scale vel or at no very 

 great depth. Borers and nestlers may also be indicative of faults and 

 disconformities, and may even constitute th6 only relics of the fauna of 

 the region where they once lived. 



In the history of deposition in a given locality, the full significance 

 of the former existence of an exposed ledge of rock, as evidenced by the 

 occurrence of fossil borers, must depend, however, upon information con- 

 cerning the faunas of the beds in question and the sequence of faunas, 

 both above and below, the texture of the rocks, their stratigraphic rela- 

 tions and correlation with other beds, and the recurrence of similar se'i^s 

 of conditions over a considerable range of territory. 



