The habits and distribution of this marine molluscan pest chiefly concern the engineer and ship-builder at the present day, but in the past their presence is a considerable guide to the geologist as to the conditions under which many of the Eocene beds were deposited. It does not seem to have existed in the Gault sea, for though the driftwood of this period is riddled by some of its destructive ancestors, such as Teredina, Lamarck, and perhaps Xylophaga, it is far less completely destroyed than when the true ship-worm has been at work. Something very like it is present in the Chalk; but when we come to the Eocenes, there is no longer any possibility of doubt as to the animal which has made its home in the drifting logs, for they are riddled in the way peculiar to the true ship-worm, and its shell is frequently left in the bore-holes