Page:Parasaurolophus walkeri, a new genus and species of trachodont dinosaur.pdf/13

Rh It must be understood that the above sutures are very uncertain and that, as far as real evidence goes, the whole of the superior surface of head and crest is composed of premaxillaries. Maximum width of premaxillaries at duck-bill, 170 mm.; width of contracted part behind duck-bill, 125 mm.; width of narial opening, 80 mm.; tip of premaxillary to posterior suture with nasal (?), 460 mm.

Nasal—As already stated the sutures with the premaxillary are problematical; it is reasonable, however, to assume that the nasals occur, and if so, they must form the major part of the superior surface of the head and of the crest. A median sutural line can be observed superiorly, and a deeply incised suture separates the nasal (and premaxillary) from the maxillary amd from the bone which forms the lower half of the crest, and which is herein provisionally regarded as the frontal.

Crest (Plate IV; Plate V, Fig. 1; text Figs. 1 and 2)—If the interpretation above is correct the crest is formed of nasals and frontals; it fades imperceptibly into the superior surface of the head proper; inferiorly it may be said to terminate at a notch behind the orbit which it is proposed to call the postorbital notch. The length from this notch to the inferior-posterior end is 875 mm. and to the superior margin 200 mm. The latter ﬁgure is doubtless too high as it makes no allowance for the downward shearing which the skull has suffered.

Posteriorly the crest is subquadrate in cross section, but farther forward it is drawn out to a keel inferiorly; just behind the postorbital notch it is sub-trapezoidal owing to a widening of the inferior surface. The distal part is expanded rather abruptly for a distance of about 20 mm. from the end. The posterior surface of this expansion is strikingly flat, is inclined downwards and backwards from the axis of the crest, and measures 90 mm. transversely and 110 mm. vertically. Laterally, this terminal expansion is sharply deﬁned against the nasal, but less distinctly against the frontal. On the superior surface a median suture is plainly discernible almost to the extreme end. The strongly marked