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IV] seems little doubt that some of them were actually torn off by the trawl. One block sent to me was full of recently dead half-grown Pholas parva, all of one age, and must evidently have been torn off the solid ledge. Pholas never makes its home in loose blocks. We unfortunately know very little about the natural history of the boring mollusca and their length of life. If, as I think, this species takes two years to reach full growth, then it is evident that the ledge of moorlog full of half-grown specimens must have been exposed to the sea continuously for one year, but not for longer. It ought also perhaps to tell us the depth of water from which the mass was torn; but nothing is known as to the depth to which Pholas may extend—it has the reputation of occurring between tide-marks or just below, but it may extend downwards wherever there is a submarine cliff.

Though we are still unable to locate exactly these submarine ledges or fix their depth below the sea, the blocks of moorlog are so widely distributed around the Dogger Bank, and have been dredged in such large masses, that it seems clear that a "submerged forest" forms part of the core of the bank. As nothing else approaching to a solid stratum appears to be dredged over this shoal, we may assume that the moorlog forms a sort of cap or cornice at a depth of about 10 fathoms, overlying loose sandy strata, and perhaps boulder clay, which extend downward to