SKULL OF A HADROSAURIAN DINOSAUR — GILMORE 483 



bones meet in grooved siitural contact. Ventrally the siipraoccipital 

 presents two sutiiral surfaces that articulate with the exoccipitals. The 

 posterior one is horizontal, the anterior face oblique looking outward, 

 forwa:'d, and downward. On the posterolateral angles of this bone 

 are raised smoothly rounded protuberances that look upward and 

 articulate with a cupped surface developed on the lower border of the 

 squamosal, as shown in figure 30. This rounded articular surface is 

 contributed to laterally by the exoccipital. 



This is a most unusual cranial articulation that gives every indica- 

 tion of being a movable union, although the other articulating surfaces 

 of both squamosal and supraoccipital are through the medium of 

 roughened sutural contacts. Tliis ball and socket articulation may 

 be present in all hadrosaurian cranii, but through coalescence no trace 

 of such a union has before been observed in this family or for that 

 matter in other dinosaurian skulls. The ventral side of the supra- 

 occipital, although slightly excavated at the center (see fig. 29), 

 presents a continuous roughened sutural surface across the entire 

 width of the bone, indicating that the exoccipitals meet on the median 

 line and thus exclude the supraoccipital from participation in the 

 boundary of the foramen magnum, as in Bactrosaurus} Tliis inward 

 median extension of the one exoccipital bone present is broken off, 

 so one has to rely on the continuous sutural surface as evidence of the 

 condition described above, although corroborative evidence is furnished 

 by the cross section of a skull figured by Brown,^ which shows the 

 exoccipital below the supraoccipital on the midline above the foramen 

 magnum. This is a structural modification known at this time only 

 in the hadrosaurian skull among the Dinosauria. Anteriorly the 

 supraoccipital is deeply excavated, thus forming the posterior portion 

 of the brain case. A heavy triangular-shaped sutural surface on the 

 anterior border that looks forward and outward is the contact for the 

 prootic. 



MEASUREMENTS OF SUPRAOCCIPITAL 



Greatest length 52 mm 



Greatest transverse width 65 mm 



Greatest vertical depth 42 mm 



Squamosal. — The squamosals are separated on the median line by 

 the interposition of a backwardly extended process of the parietal, 

 which they meet by a strongly ridged and grooved suture. They 

 are only 10 mm apart (see fig. 30). Tliis union with the parietal 

 continues downward and forward on either side of the upper median 

 part of the supraoccipital. Ventrally it unites wdth the supraoccip- 

 ital by a smooth cupped articulation, which rests upon the ball-like 



1 Gilmore, Charles W., Bull. Amer. Mus. Nat. Hist., vol. 67, p. 55, fig. 22, 1933. 

 » Brown, Barnum, Bull. Amer. Mus. Nat. Hist., vol. 33, pi. 36, 1914. 



