966 A. L. BARROWS FOSSIL ROCK-BORING ANIMALS 



We must know particularly whether the holes in which these fossils may 

 be found were made by the animals themselves or by other animals in 

 firm or indurated rock or in compact sand, mud, or clay. Burrowers in 

 mud or sand are found under a great variety of conditions, and the pres- 

 ence of tliese burrowers usually indicates a continuity rather than a cessa- 

 tion of deposition. The evidence offered by rock-borers, however, suggests 

 the occurrence of a period of erosion between periods of deposition, and 

 it is, therefore, with the true rock-borers and the characters which dis- 

 tinguish them from mud and sand burrowers that we are here especially 

 concerned. 



Borers into rock are found among all of the principal invertebrate 

 phyla, and include sponges of tlie genus Gliona, several types of marine 

 worms, sea-urchins, crustaceans, and mollusks.^ Of this whole array of 

 boring animals it is the fossil sea-urchins and certain pelecypod mollusks 

 which are of especial significance, because the holes made by these ani- 

 mals are of a size not likely to be obscured, and because the pelecypod 

 boring mollusks, at least, have occurred in abundance since Middle Meso- 

 zoic times. 



RoCK-BORING SeA-UEOHINS 



Among the rock-dwelling sea-urchins but few boring species are known, 

 and these belong largely to the genera Echinus and Strongylocentrotus. 

 These sea-urchins are often found on the exposed parts of reefs where 

 the waves beat with violence. It seems probable that the wash of the 

 waves may at first have caused the ventral spines of the sea-urchins to 

 have rasped away the rock to which the urchins clung, and that thus the 

 capacity for eroding the rock might have been initiated quite acciden- 

 tally, developing later into systematic movements on the part of the 

 urchins, and finally resulting in the excavation of a hole and escape from 

 the fury of the surf. Sea-urchins boring under such a stimulus are thus 

 associated with definite conditions of exposure to the sea on a more or 

 less open coast, Avith location at or near tide levels and attachment to 

 hard reef rock. The bores of sea-urchins of the genus Strongylocen- 

 trotus, for instance, are large and cup-shaped, with openings but slightly 

 smaller than the neatest diameter of the hole. 



- At the reading of this paper at the Albany meeting of the Paleontological Society, 

 it was brought out in discussion that certain types of marine algse are linown to riddle 

 limestone, and especially the calcareous shells of mollusks, Avith minute holes, and that 

 certain worms are also frequent borers into rock. The writer regrets, however, not to 

 be sufficiently familiar with the habits or characters of marine worms to be able to sug- 

 gest a basis for distinguishing between those which may be regarded as habitual borers 

 into indurated rock and those which construct slime tubes in beds of sand or mud. 



