THE SKULL. 



21 



abuts, while the broad, thin end. of the quadrate is wedged in between the exoccipital process 

 and the anterior end of the squamosal and is rather firmly held in a groove on the under surface 

 of the latter. The position occupied by this groove for the quadrate, as well as the form of the 

 squamosal, varies greatly in the different genera and species. On the inferior side of the squa- 

 mosal, just posterior to the transverse buttress for the exoccipital process, there is a broad, 

 shallow, concave surface, while be- 

 yond this the inferior surface of the 

 bone is convex throughout. The 

 line marking the boundary between 

 these convex and concave areas is 

 approximately that which separ- 

 ates the external free portion of the 

 squamosal from the internal por- 

 tion, which in life was embedded 

 in flesh. The dividing line between 

 these two areas is indicated by a 

 difference in texture and surface 

 markings, that of the external and 

 free area being much more rugose, 

 especially in old individuals, where 

 it was probably covered with a 

 horny substance similar to that 

 which invests the skulls of recent 



turtles. In the skulls of young individuals like that at present under consideration (No. 1821, 

 Yale Museum), these two surfaces are less distinctly differentiated than in the skulls of older 

 individuals, though still quite apparent, as shown in figs. 6 and 15. Marsh has considered the 



Fig. 14.— 1, Right squamosal of Torosaurus gladius (type), No. 1831, Yale Museum^ 

 inferior view. 2, Right squamosal of Triceratops flabellatus (type), No. 1824, Yale 

 Museum; inferior view. 3, Right squamosal of Ceratops montanus (cotype),No. 

 4802, U. S. National Museum; inferior view, e, Epoccipital; s, parietal surface; q, 

 quadrate groove. All one-twentieth natural size. After Marsh. 



Fig. 15.— Inferior view of right squamosal of Triceratops flabellatus (type), No. 1821, Yale Museum, e, Epoccipital; sp, surface for contact 

 with parietal; ex, buttress for exoccipital; qg, quadrate groove; p, process overlapping quadratojugal; I, margin forming posterior border 

 of lateral temporal fossa; pfs, postfrontal border; jp, process interposed between jugal and postfrontal; exs, surface overlapped by exoc- 

 cipital process. One-eighth natural size. 



partial absence of this character as of generic importance, and has made it distinctive of the 

 genus Sterrholophus, of which the present specimen was the type. To the present writer it 

 would seem, however, that the difference in the rugosity of the external surface of the squamosal 

 is due rather to a difference in the age of the different individuals, while it may also have been, 

 in part at least, a sexual character. In support of this view it may be mentioned that the 

 rugosities on this surface are less distinct in those specimens where the sutures are less obliterated 



