THE SKULL. 



23 



A B 



Fig. 17. — A, Internal view of right quadrate of type of Triceratops ftabel- 

 latus, No. 1821, Yale Museum; B, external view of same, sq, Surface 

 fitting into quadrate groove of squamosal; pt, surface overlapped by 

 pterygoid; ptn, notch for process of pterygoid; sa, surface for articu- 

 lation with lower jaw; qj, surface for contact with quadratojugal. 

 One-eighth natural size. 



underlapped by the exoccipital process. On the internal side near the superior border and 

 toward the anterior end of the quadrate there is a deep pocket into which fitted the posterior 

 process of the pterygoid, while the thin posterior blade of the pterygoid overlapped the delicate 

 superior angle of the quadrate when these bones were in position, as shown in fig. 24, from 

 the type of Triceratops horridus. The external border of the quadrate anteriorly, throughout 

 half its length, is opposed to the quadratojugal. The latter element sends backward a keeled 

 process, which curves round the inferior border of the thickened shaft of the quadrate and is 

 continued upward and backward along the 

 external surface of that bone, finally com- 

 ing in contact with and passing beneath a 

 sharp-pointed anterior projection from the 

 squamosal, as is beautifully shown in PI. 

 XXVII, from the type of Triceratops serra- 

 tus, No. 1823 of the Yale Museum collection. 

 Marsh has stated that the quadratojugal 

 does not articulate with the squamosal in 

 the genus Ceratops, and has considered this 

 as a character distinguishing that genus 

 from Triceratops, in which genus he states 

 that these two elements are firmly united 

 with one another by sutures. While there 

 is a considerable difference in the character 

 of the union of these elements in these two 

 genera, the union being apparently much 

 stronger in Ceratops, it is clear that they do 

 articulate with one another in Triceratops. 



The articular surfaces for the lower jaw on the quadrates are in nearly the same vertical 

 plane as the occipital condyle in the Ceratopsia instead of being placed far back as in the 

 crocodile. 



The quadratojugal, fig. 18, A andB, is irregularly triangular in outline, very thick medially, 

 but thin about the margins. It is wedged in between the jugal and quadrate and also has, 

 as stated above, a limited contact with the squamosal. The squamosal and quadrate articula- 

 tions have already been described and need no further mention here. The surface for articula- 

 tion with the jugal is irregularly elliptical in outline and occu- 

 pies the anterior half of the external surface of the bone. It 

 bears a number of deep grooves and intervening ridges, which 

 correspond to similar inequalities on the opposing surface 

 of the jugal, thus giving great strength and rigidity to this 

 region. The quadratojugal is the smallest of the three ele- 

 ments which in the Ceratopsia form the quadratojugal arch. 

 The jugal is the largest bone of the three entering into 

 the construction of the quadratojugal arch. Seen from the 

 side when in position it appears T-shaped, the stem being 

 formed by that portion of the bone which overlaps the quad- 

 ratojugal, as shown in fig. 10 and in Pis. XXVII and XXXIV. 

 The posterior branch of the crossbar at the top sends back- 

 ward a rather slender bar which interlocks with an anterior projection from the squamosal, as 

 shown in PL XLIV, and forms most of the supero-external border of the lateral temporal fossa. 

 Anteriorly and superiorly the transverse portion of the jugal articulates with the maxillarj- and 

 sends forward a narrow process which is interposed between the lachrymal and the superior bor- 

 der of the maxillary. At the extreme top the jugal forms the anteroinferior border of the orbit, 

 inclosing somewhat less than one-third of that opening. Antero-superiorly it articulates with 



Fig. 18.— A, Internal view of quadratojugal of 

 type of Triceratops flabellatus, No. 1821, Yale 

 Museum; B, external view of same, es, Free 

 surface; sq, surface for quadrate; sj, surface 

 for jugal. One-eighth natural size. 



