The borings of the Polychæt Annelid Polydora ciliata (Johnston) are familiar in limestone pebbles and the stouter mollusc-shells on our coasts, but shales and occasionally other rocks are not free from their attacks. They may be recognized by the double aperture, often loosely described as keyhole-shaped, leading to a U-shaped cylindrical tube. The union of the two apertures in a single depression, by the loss of the organically formed septum between them, produces a more slit-like opening, whereas if the surface of the pebble be further worn the two round holes are more clearly distinguished. The differences between the burrows of Polydora and those of other lithodomous Annelids, especially the associated Dodecaceria, were clearly stated by Dr. W. C. M'Intosh in a very full paper “On the Boring of certain Annelids” (1868, Ann. Mag. Nat. Hist. [4], ii, pp. 276–95, pls. xviii–xx).