Holotype. Although the exact horizon of the holotype is not known, Arkell (1944, p. 290) noted that “P. williamsoni is said to be characteristic of the lower part of the Passage Beds of Scarborough and Filey (see Fox-Strangways, 1892[a], p. 318)” and he recorded other specimens from the Passage Beds [Member] at Scarborough.
Colour Laser Scan.