SEA-UHCHINS AND PELECYPODS 967 



EOCK-BORING PELECYPODS 



The peiecypods which are known to bore only into rock fall into two 

 groups : Mytilids oi' the genera Adula and Lithodomus, which probably 

 use a solvent as the principal agent in boring, and pholads of such highly 

 specialized genera as Pholadidea and Farapholas, which bore by grinding 

 away the rocks with their shells. In addition to these there are other 

 genera, species of which, exhibiting but slight morphologic differences, 

 are known to burrow into sand or clay, to bore into rock, or to bore into 

 both rock and sand or mud or clay. These include Clavagella and Gas- 

 troc/iwiia of the Gastroch^enida^, Flati/odon of the Myacidse, and PJiolas 

 and Zirplicea of the Pholadida3. Since these latter genera offer no marks 

 by which it is possible to determine whether they may have bored only 

 into mud, or at times also into mud, sand, or clay, fossils of these groups 

 can not be relied upon to indicate the nature of the substratum into which 

 they bored. All the true rock-borers seem to carefully select the rocks 

 into which they enter, attacking readily limestone and shale and fine 

 sandstones, even though bound by a strong cement, and avoiding con- 

 glomerates and the harder metamorphic rocks unless the latter are par- 

 tially disintegrated by exposure. It is, of course, when boring into such 

 sedimentary rocks as sandstone and shale, rather than into igneous rocks, 

 that rock-borers can be of the greatest use in throwing light on ohscure 

 stratigraphic relations. 



Nestling Pelecypods 



In the study of the fossil rock-boring pelecypods the mode of origin of 

 the boring habit and the method of boring throws certain light on the 

 reliability of suspected rock-borers as determiners of the state of the 

 substratum at the time when they lived. It seems probable that the 

 waves may have caused the boring mytilids Adula and LiiUudoiiiufi, mem- 

 bers of a reef-dwelling group of pelecypods attached to the rock by means 

 of a byssus, to grind out cavities, which may have become enlarged by 

 solution of the rock due to the accumulation of acidic organic excretions. 

 When nestling in natural crevices, these excretions may also have had a 

 similar solvent effect in reshaping the cavity to fit the shell. Species of 

 Mytilus living on exposed reefs illustrate today certain phases of just 

 such a possible development of the boring habit. At all events, to judge 

 from the shape of the holes of the mytilid borers, which conform accu- 

 rately to the sliape of tlie shells, it seeins probable, though not yet demon- 

 strated, that these borers do soci-ete a solvent effective either in dissolvino- 



