Scolithus, a supposed fossil burrowing worm of the arenico-la family, whose long vertical holes are very common in the Potsdam sandstones, of the lower Silurian period. These holes, now filled with rocky material, were for a long time believed to be the remains of a fucoid plant, and afterward until recently to indicate the existence of a long marine worm, which inhabited the sands not far below tide level. The most common form has been named S. linearis, and, whatever it be, is one of the earliest fossils. Since attention has been paid to the habits of sponges, especially to those of cliona and its allies, which mine and perforate shells, some palaeontologists are disposed to attribute the sco-lithus marks to these rather than to marine worms. Sponges are known to have existed at the period of the Trenton limestone, next above the Potsdam sandstone, and it is highly probable that most if not all of these burrows are due to ancient mining sponges.