On seacoasts today, especially in tropic and temperate regions, animals are found boring into rocks which have been formed at various periods from recent to those of early geologic date. Related forms are frequently found as fossils in formations which are overlain by beds attributed to late Mesozoic or to Cenozoic times. The peculiar habitat of borers suggests that acquaintance with the conditions under which the recent forms live may offer distinct contributions to our knowledge of the depositional history of formations in which the fossilized forms are found. It is, however, of primary importance to be able to distinguish with certainty the special characters-of each of the large number of marine animals which seek refuge in natural crevices or in holes, in order to be able to judge correctly the habits of fossils suspected of being rock-borers.
We must know particularly whether the holes in which these fossils may . . .