Diagnosis. - Tubular, U-shapeded borings, probalblly left by polychaetes, round in section and about 6 to 8 mm in diameter. The limbs of the U~shaped loop are usually 3 to 15 cm long, thIis length being sometimes shortened as a result of abrasiono (Fig.3c). The space between the limbs aboult 1.2 cm to 1.8 cm wide camposed of an original, non-reworked, solid rock. Boring openings on the surface round, without a funnel-like depression. In the near-opernng part, the limbs display a characteristic defllection of their trace in relation to the deeper parts of the boring.