Family Pholadidae are the piddock clams, which bore into shale, clay, or firm mud. Much of the anterior portion of the shell is roughened so that the animal can rasp a hole in the rock or clay much like an augur bit (photo). The anterior portion of the shell, while higher and more globose than the posterior portion, is not nearly globular. In this species, the anterior portion occupies less than half the length of the valve and is separated from the posterior in a well-defined manner by a groove which runs from the dorsal to the ventral side. It has a myophore (apophysis) in both valves (photo). The posterior end of the shell, though narrower than the anterior, does not taper to a point like a bird's beak. In small individuals there is a gape between the valves at the anterior end for the foot to protrude, but in a full-grown specimen such as the individual above the gape has been covered over by a calcareous callum (photo). It has no calcified siphonoplax. The anterior end has a thick, shieldlike plate (protoplax) dorsal to the anterior rasping portion (photo). The umbonal reflections (the calcified plates between the protoplax and the rasping portion of each valve) is tightly applied to the shell (as opposed to being free from the shell for part of their length, especially at the anterior end) (photo).
How to Distinguish from Similar Species: Penitella gabbi has umbonal reflections free from the anterior rasping part of the shell for about half the length of the reflection. Penitella conradi and Penitella penita have a siphonoplax. Zirfaea pilsbryi has an anterior rasping portion about half the length of the valve and with no protoplax, plus its anterior gape does not close. Netastoma rostrata has no myophores. The anterior portion of Barnea subtruncata is not clearly set off from the posterior portion by a groove. Parapholas californica lives farther south, its valves are clearly divided into three sections, and it has distinctive overlapping plates of periostracum on the posterior end.