SKULL OF A HADROSAUEIAN DINOSAUR — GILMORE 



485 



MEASUREMENTS OF SQUAMOSAL 



Length from posterior border to anterior termination 



beneath the postfrontal 80 mm 



Breadth from parietal contact, obliquely outward and 



downward to end of external process loO mm 



Depth from supratemporal fossa border to rim of cotylus.. 52 mm 



Eioccipital. — Only the left exoccipital is present, but it is in excel- 

 lent preservation, as showTi in figure 31. The exoccipital contributes 

 extensively to the formation a 



of the occipital condyle; a 

 heavy posterior projection 

 contributes nearly one-third 

 of the complete condyle. 

 The inferior horizontal surface 

 unites with the basioccipital 

 by a coarsely roughened su- 

 tural surface. Internally the 

 surface is concave dorsoven- 

 trally forming the lateral 

 boundary of the foramen 

 magnum. This portion of 

 the exoccipital is perforated 

 by two foramina, wliich pass 

 diagonally through the bone. 

 The larger and more posterior 

 is for the passage of the 

 twelfth or hypoglossal nerve; 

 the smaller anterior one for 

 the eleventh or accessory 

 nerve. The upper portion 

 of the exoccipital has been 

 described in connection with 

 the supraoccipital. 



The outer portion of this 

 bone extends outward and 

 backward and develops a large 

 hooked paraoccipital process 

 that in position Ues against the squamosal process, which it supports 

 and resembles in general shape. Its bluntly pointed extremity extends 

 below that of the squamosal. On the inner end of the anterior side 

 below the articulation of the supraoccipital a grooved subtriangular 

 surface marks the union with the prootic (see fig. 31, B, Pro). 



Frontals. — The coossified frontal bones are shown in figure 32. 

 The frontals are of the typical Lambeosaurinae type; that is, much 

 reduced in length, wider than long, and with a deep nearly vertical 

 sutural surface for union \\dth the nasals. This sutural area is nearly 



Pro 



Figure 31— Left exoccipital (U.S.N.M. no. 11893): A. 

 Posterior view; B, anterior view. Oc, exoccipital con- 

 tribution to the occipital condyle; Pro, sutural surface 

 for union of prootic; Soc, articulating surface for union 

 with the supraoccipital; XI and XII, foramena for 

 the eleventh and twelfth nerves. One-half natural size. 



