Page:Quarterly Journal of the Geological Society of London, vol. 34.djvu/877

Rh rior border is mutilated), this os articulare maybe described as consisting of a vertical plate largely appearing in the outer surface of the jaw, and of a horizontal plate projecting inwards from the former in a shelf-like manner, bearing the tympanic articular surface.

Vertical plate.—The outer surface of this is smooth; so much of it as lies in advance of the tympanic joint has a roughly quadrilateral outline (see fig. 1); and here its vertical depth is about double that of the part behind it abreast of the tympanic joint, which narrows chiefly by the sharp descent of the upper border of the plate. Below, the quadrilateral part is bent inwards, which makes its upper border overhang, particularly above a large foramen to be shortly described. Abreast of the tympanic joint the surface of the plate bends inwards, through the tapering-off of the hinder end of the jaw.

At the bottom of a shallow circular pit, rather behind and above the middle of the quadrilateral part of this plate, is a large foramen, which would give passage to a cedar drawing-pencil (fig. 1, b). Its inner opening is deeply placed beneath and hidden by a thin lip or sheet rising from the front of the tympanic surface (figs. 1, 2, 3, c).

Figs. 1-4. — Htght Os articulare o/Iguanodon, one third natural size. Fig. 1. — Outer View.

a. Tympanic joint, b. Foramen, c. Ascending process from tympanic surface. Fig. 2. — Tympanic Surface, from above.

Ascending process.