E-Fauna BC: Electronic Atlas of the Wildlife of British Columbia

This white species is up to 9 cm long and about 1 cm thick at its midpoint. The body is usually V-shaped, fat in the middle and long and tapered at each end. The skin is stiff with ossicles. The non-retractile tube feet are in five distict bands, with two to four rows in each. The tube feet tend to be reduced in size and number toward the anterior and posterior ends of the body. Two of the ten tentacles are reduced in size. The tentacles and mouth area are speckled brown.

Skin ossicles: round to triangular perforated plates (100-300 μm) with a large central spire covering half the width of the plate; curved supporting tables in the tube feet have a low bumpy spire. Tentacle ossicles: in two forms; finer, curved, elongate, oval plates and large, more robust rods with a few holes.

Pentamera rigida is similar to Pentamera populifera and usually combined with it in collections. Pentamera rigida is more elongate, thinner and stiffer. The table ossicles have a low, broad central spire and the supporting rods have a low spire. In contrast P. populifera has skin ossicles with a tall, narrow central spire and supporting rods with a tall spire. Most previous references have lumped this undescribed species in with either P. populifera or P. pseudopopulifera found in southern California. Pentamera rigida is also similar in general appearance to Pentamera pediparva, but the skin ossicles are quite different.