The genus is closely related to, possibly identical with, Œstocephalus, but additional material will be required to settle this point.
Cope ([123]) gives the following key for the separation of the 5 species:
| x. | Abdominal rods coarser, not more than 10 in 5 mm. | |
| Median pectoral shield discoid, radiate-ridged; muzzle short | [P. nummifer] | |
| Median pectoral shield oval, pitted and ridged | [P. marshii] | |
| xx. | Abdominal rods hair-like, 15 or more in 5 mm. | |
| Median pectoral shield with radii from the center, the principal forming a cross; form wider | [P. vinchellianus] | |
| Middle pectoral with pits at the center and few or no radii; form narrow | [P. pectinatus] | |
| Middle pectoral shield narrow, closely reticulate medially, and radiate towards the circumference; size half that of last | [P. serrula] |
Cope, Proc. Acad. Nat. Sci., 1868, p. 216.
Cope, Trans. Am. Phil. Soc., XIV, p. 20, 1869.
Cope, Trans. Am. Phil. Soc., XV, p. 266, 1874.
Cope, Geol. Surv. Ohio, II, pt. II, p. 377, pl. xxvii, fig. 7; xxviii, figs. 2, 3, 6; pl. xxix, fig. 2; pl. xxx, fig. 2; pl. xxxv, figs. 1-3; pl. xli, fig. 1, 1875.
Moodie, Proc. U. S. Nat. Mus., 37, p. 24, pl. 8, fig. 3, 1909.
Schwarz, Beiträge zur Paleontologie und Geologie Osterreich-Ungarns und des Orients, Bd. XXI, p. 83, figs. 23, 24, 26, 1908.