Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/610

524 to be the edentulous beak-like part of the præmaxilla, known to us by Mr. Fox's skull. That which I judge to be the trenchant border has the same length as the edentulous part of the mandible.

Teeth.—The other remnants of upper and lower jaw, though very fragmentary, throw new light on the dentition of Hypsilophodon, so that, with this and several teeth which had fallen out and were recovered from the clay by washing, I can now illustrate nearly every phase in the life of a tooth, from the immature crown, which had not come into use, to the worn-out stump.

The right mandible just described had at least ten mature teeth in use, of which the crowns of four only remain; and these exhibit the characters of the maxillary teeth of Mr. Pox's skull. The last tooth is smaller than those immediately before it. The 2nd-6th, counted from behind forwards, were larger than the four next preceding them. The crowns of three of these are broken off and lost; but one remains. It is worn nearly to the root; and a young unused crown rising up at the inner side of this stump shows these teeth to have corresponded in shape and size to the anterior smaller maxillary teeth in Mr. Fox's skull. Between the foremost of these teeth and the edentulous extremity of the jaw, the outer parapet of the dentary bone has been broken away: no teeth remain here; but I fancy I can discern traces of three alveoli suitable for the reception of the roots of teeth of the cylindrical form, such as are present in the hinder part of the præmaxilla. In the clay, at a short distance, lies one such tooth; and near this is the impression of another. As in Mr. Fox's skull, so here, there are two forms of tooth—one simple and cylindrical, the other ornate and strongly compressed.

Cylindrical teeth (fig. 7).—A perfect, mature tooth of this sort measured ⋅4 inch; of this, nearly ⋅15 belong to the crown, which is separated from the root by a slight constriction or neck. The root is slightly contracted towards each end, and dilated in the middle; its cross section is nearly circular, and its surface is smooth. Two, which I slit longitudinally, had a very large pulp-cavity filled with spar. The crown is slightly and unequally compressed, the inner contour of its cross section being slightly more convex than the outer. Its apex is acuminate, and is slightly inflected, which renders the outer longitudinal outline convex, and makes the inner one sinuous, concave near the point, and convex towards the root. The outer and inner surfaces meet angularly, making a low wing, within which and parallel with it, upon the inner surface, is a minute shallow longitudinal groove. In very perfect unworn crowns, the marginal wing bears a row of minute tubercles, just visible in a strong light to the unaided eye. Both surfaces are highly polished and smooth; upon the outer a few very minute longitudinal striæ are discernible. Towards the neck the surfaces are beset with excessively minute tubercles (not recognizable as such without a magnifier), the collective effect of which to the unaided eye is an extremely fine wrinkling.

Compressed sculptured Teeth (figs. 4, 5, 6).—Both the varieties described by Prof. Huxley from Mr. Fox's skull are amply illustrated by my specimens, the smaller variety occurring in the front of the