C. placentula is a small epiphytic diatom which often grows on strands of Cladophora. Its raphe and rapheless valves both have small radiate striae, but the areolae on the rapheless valve tend to be coarser and more lineolate.

P. placentula can sometimes be confused with P. pediculus. P. placentula valves tend to be more elongated and flatter, while P. pediculus sends to be fatter and more concave/convex across the apical axis.

In plate, the left and center specimens are of the raphe valve; the specimen on the right is a rapheless valve.