« Shell inequilateral, elliptical. Ribs and interstices about equally broad, shining through at the inside. Hinge-plate regularly curved. » – J. J. van Aartsen & J. Goud: “Notes on Lusitanian species…”, Basteria 64, 2000.

« The main difference between P. vroomi and P. scriptum is found in the hinge. In P. vroomi the hinge is weak and the angle between the dorsal side and the anterior lateral(s) is about 140°. In P. scriptum the hinge is stronger and the angle between the dorsal side and the anterior lateral(s) is larger, viz. 160°. As a consequence, the the hinge of P. vroomi looks more curved compared to that of P. scriptum. » – Ibid.