488 PROCEEDINGS OF THE NATIONAL MUSEUM vol.84 



forms it comprises a small portion of the border. The characteristic 

 shape of this bone is well shown in figure 33. 



Prootic. — Both disarticulated prootic bones are present, the left 

 element in excellent preservation. The bone here called prootic in 

 all probability represents the coossified prootic, epiotic, and opisthotic, 

 certainly the last, for as in most reptiles these probably fused early 

 in life, and thus all trace of their sutural junctions has long since been 

 obliterated. 



The prootic lies posterior to the alisphenoid and anterior to the 

 exoccipital. Above it is in contact with the supraoccipital, below 

 with the basisphenoid. In Edmontosaurus, Lambe * shows the 

 prootic in contact wdth the parietal, and likewise Brown " in a tra- 

 chodont brain case described by him found this complex in contact 

 with the parietal. A similar condition exists in the crocodile skull. 

 However, in the skull under consideration they are distinctly sepa- 

 rated by the interposition of portions of the supraoccipital and 

 alisphenoid bones. The pointed posterior half of this complex, 

 probably the opisthotic portion on the inner side, presents two 

 longitudinally ridged and grooved sutural surfaces, the upper one 

 uniting with the supraoccipital, the lower with the exoccipital. This 

 side is about equally divided between the two bones and overlaps 

 the junction of the supraoccipital and exoccipital. The prootic proper 

 is perforated by the foramen for the seventh or facial nerve. The 

 anterior border is deeply notched by the foramen ovale for the fifth 

 or trigeminal nerve, but the portion carrying the foramen for the 

 eighth or internal auditory meatus is missing, and its position cannot 

 be accurately determined in this specimen. 



Alisphenoid. — The aHsphenoid has the usual triangular curved 

 form and lies in front of the prootic and bounds the large foramen 

 for the trigeminal nerve in front. It connects superiorly with the 

 parietal and frontal. The outer rounded end is received in a pit on 

 the upper inner surface of the postfrontal+postorbital complex. 

 This bone forms the wall of the brain case, which lodges the cerebral 

 hemispheres. 



The division between the alisphenoid and prootic is marked by a 

 suture that descends from the floor of the supratemporal fossa and 

 enters the foramen ovale forward of the upper curve of that opening. 

 The external surface forms a part of the inner and anterior boundaries 

 of the supratemporal fossa. A narrow groove on the external sur- 

 face extending forward from the foramen ovale was for the reception 

 of the ophthalmic branch of the fifth nerve. In form and relation- 

 ships with surrounding elements this alisphenoid is in full accord 

 with the conditions found in other hadrosaurian skulls. Anteriorly 



« Lambe, L. M., Geol. Surv. Canada Mem. 120, p. 17, flg. 26, 1920. 

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



