#piddock

AKA Angelwings.

As the sign under the fossil says, not every shell in the cliffs along the fossil coast are actually fossils, although some have certainly been there a good while, because they dug their way into the soft rock before the entire area got pushed back up above sealevel.

Pholadidea and the other genera in the family twist their shells back and forth to excavate tubular burrows in clay, peat or soft rock. Despite the hard work involved, the shells are actually quite fragile, and difficult to collect intact. Pholas dactylus, the common piddock, can not only manage the same trick in harder rock, but is also bioluminescent.

Pleistocene piddock bore holes

Monterey formation shale from Farm Center, Carmel Valley, CA

Pleistocene rock with piddock (boring clam) holes

AKA Parapholas janellii, Pholas californica, and Pholas janellii.

A rock-boring clam found in shale, mudstone, chert and hard sand along the Pacific coast of North America. They line the tunnel with a chimney made of small rock fragments glued together with calcium carbonate, and the chimney may protrude a short way into the water column.