Page:Quarterly Journal of the Geological Society of London, vol. 29.djvu/611

1873.] series, and the larger form occupying a posterior position. In both varieties one surface of the tooth (that towards the cavity of the mouth in the lower jaw, and the outer in the upper jaw) is exquisitely sculptured by longitudinal ridges passing from a raised cingulum at the junction of crown and root to the free border of the crown. The general outline of the crown is subrhomboidal. Both surfaces, longitudinally and transversely, are convex. In a nearly perfect tooth of the smaller variety (fig. 4), which had only just come into use, the cingulum forms an angle open towards the summit of the crown. The principal ridge runs from the open nearly axial angle to the trenchant border, and it forms the apex of the crown. At each side of it is a small secondary ridge; one of these does not quite reach the cingulum. Between the free ends of these minor ridges, which give this part of the border of the crown a coarse serration, and the lateral terminations of the cingulum, the sides of the crown are very finely serrated, recalling in miniature the marginal serrature of the teeth of Iguanodon Mantelli. In the larger variety the ornamented surface of the crown is sculptured by a greater number of ridges; and these are more equal in size (fig; 6). Some of them divide near the trenchant border of the crown, rendering this, when unworn, beautifully crenated; and the sides of these larger teeth are beautifully serrated, as in the smaller variety. The cutting-border of these larger teeth, before it has been worn, is rounder in outline and less angular than that of the smaller ones. The unridged surfaces of the crowns bear a few very minute inconspicuous striæ. All crowns which have risen above the crest of the outer parapet of the jaw bear marks of wear. They are obliquely ground; the sculptured surface remains longest, and it forms a cutting-edge, which is at first serrated by the cross sections of the longitudinal ridges, but later becomes merely sinuous as these grow less prominent in the level of the lateral angles of the crown. The maxillary tooth of Mr. Fox's skull, figured in pl. 1. vol. xxvi. Quart. Journ. Geol. Soc, is thus worn. The worn surface of large crowns is marked by slight elevations not deserving the name of ridges, running between the inner and outer surface; and the attritional striæ, which are discernible in all worn teeth, have the same direction. By the time that the crown has become worn down to the level of the outer border of the jaw, the long cylindroid fang also has nearly disappeared, so that very slight force would detach the remnant of a tooth in this condition. The successional teeth rise at the inner side of the old ones, as in existing lizards.

Attachment of Teeth.—A transverse section through the fang of a cylindric tooth in situ shows it to be contained in a distinct, separate socket. With respect to the compressed teeth, I am inclined to think that the same does not strictly obtain. As in Iguanodon Mantelli, the outer wall of the tooth-groove sends inwards partitions, which practically separate the teeth from one another, and must have afforded them a very firm support; but I doubt if these partitions actually reached the inner wall and became confluent with it.