968 A. L. BARROWS FOSSIL ROCK-BORING ANIMALS 



the rock into which they bore, if limestone, or in dissolving the cement 

 which holds the rock together, if shale or sandstone. Such an origin of 

 the boring method in these genera, and the known association of mytilids 

 with a rocky substratum rather than with sand or mud, mark these boring 

 genera, when found preserved in bores, as having lived in a rocky sub- 

 stratum. They are not to be confused w^itli mud- or sand-burr owers. 



On the other hand, it is possible that the rock-boring pholads, using 

 a mechanical method of boring by grinding the rock away with the hard- 

 ened anterior edges of their shells, are derived from a type of mud- or 

 sand-burrower, similar perhaps to Panopea, by a parallel development of 

 the habit of boring, together with the necessary morphologic modifica- 

 tions. Borers derived in this way directly from mud- and sand-burrow- 

 ers, which may have originally sought refuge from predacious enemies by 

 burrowing, probably did not pass through a nestling stage, and they may 

 be considered to have migrated under the process of development of the 

 boring habit from beach or estuarine localities to reef localities nearer 

 the open sea, where both a better food supply and a stronger and more 

 permanent domicile might be obtained. The less specialized genera 

 Zirphcea and Pliolas illustrate just such a probable transition, and have 

 not always been habitual borers into hard rock. The distribution of 

 borers derived from sand-burrowing ancestors,^ both horizontally among 

 the bays and inlets of the coast and vertically below tide level, is probably 

 wider than that of borers derived from a reef -dwelling type. 



Characteristics of Eock-borers 



The development of the habit of boring is, moreover, accompanied by 

 certain morphologic modifications. Though the shells of the rock-boring 

 mytilid species have no specialized processes by which the rock may be 

 mechanically worn away, and show but few marks of shell erosion, the 

 spindle-like shape of species of LitJtodonnis is characteristic- of a bor-er 

 and suggests the possible combination of meclianical and solvent methods 

 in this case. Among the mechanical pelecypod borers, a shape present- 

 ing a superficial radial symmetry about the longitudinal axis of the bore 

 is characteristic. Special development of the foot and valve muscles, the 

 development of a myophore, and the production of a serrated edge with 

 hardened grinding points on the anterior part of the shell occur in the 

 pholads and are accentuated in the rock-boring species. Accessory plates 

 may be added to take the place of a degenerated liinge apparatus. The 

 subspherical shape of the anterior end of tlie sliell is more pronounced in 

 the rock-boring ])ho]a(ls tliaii in those wliicli may bore into sand or clay. 



