EVIDENCE FURNISHED BY THE BOKERS 971 



feriiig inclination, though in such cases other indications of a non-con- 

 formity are usually apparent. 



Evidence furnished by the Pelecypods 



Evidence of a nature similar to that contributed by rock-borers may 

 come from a group of animals which do not make holes of their own, 

 but which, as nestlers, utilize the holes of other animals. This group 

 includes barnacles, bryozoans, and several genera of pelecypods — Tapes, 

 Cumingia, Kellia, Diplodonta, Entodesma, and Mytilus, when living in 

 exposed localities. Few, if any, of the species of these genera are inva- 

 riably nestlers; but it is when they are found in holes of other, animals 

 that they indicate that such holes must have been made in material suffi- 

 ciently firm to serve both the original maker and a subsequent tenant. 



There is also a group of pelecypods, including the genera Saxicava and 

 Petricola, in which the habit of boring seems to be variable. Certain 

 species of these genera have been reported to bore into "soft rock," often 

 into clay banks and mudstones, and others have been found nestling in 

 the holes of other borers. It seems probable that the same species may 

 in certain localities bore into the softer rock structures, and in other 

 localities satisfy its tendency for seclusion by becoming nestlers in the 

 holes of other pelecypod borers. Since their present mode of living is so 

 varied, but little confidence can be placed in these shells as indicating a 

 definite state of the substratum in which they may be found, unless con- 

 firmed by evidence from other sources. 



Conclusions 



There are, then, certain sea-urchins of the genera Echinus and 8tron- 

 gylocentrotus, members of the pelecypod genera Adula, Lithodomus, 

 Plioladidea, and Parapliolas, which at present bore habitually into indu- 

 rated rock and which do not enter less compact materials, liable to crum- 

 ble or collapse. Fossils closely related to the known rock-borers, which 

 probably use the solvent method and which are derived from reef -dwelling 

 types, may be regarded with tolerable certainty as rock-borers when found 

 fossilized in situ. Fossil borers derived from mud- and sand-burrowers 

 are also to be regarded as habitual rock-borers if possessed of characters 

 indicating a highly specialized boring apparatus, and probably have a 

 wider range of distribution horizontally and vertically along the coast 

 than borers of the former type. Morphologic modifications known to be 

 associated witli the rock-borini^ Jiabit and certain characters of the holes 



