digitation anterodorsally with the ectopterygoid and 

 posteroventrally with the symplectic, which it broadly 

 overlies; articulates by fibrous tissue ventrally with the 

 preoperculum and through cartilage posteriorly with the 

 metapterygoid. The anterior knoblike end of the quad- 

 rate articulates against the groove on the posterior edge 

 of the articular in the lower jaw, with the fibrous tissue 

 articulation strengthened by a short anteromedially 

 directed process from the anterior edge of the quadrate 

 just above its knoblike anteroventral end. 



Metapterygoid. —A large plate of bone; cartilage 

 filled along its anteroventral edge; articulates by inter- 

 digitation anterodorsally with the ectopterygoid and 

 mesopterygoid and anteroventrally with the symplectic, 

 all three of which bones the metapterygoid broadly over- 

 lies; articulates posteroventrally by fibrous tissue and 

 slight interdigitation with the hyomandibular, while an- 

 teriorly it articulates through cartilage with the quad- 

 rate. 



Opercular Region. 



Operculum. — A large flat plate with a high, well- 

 developed, lateral flange which ends dorsally by project- 

 ing over the hyomandibular to serve as a place of attach- 

 ment for muscles originating on the lateral wing of the 

 sphenotic; articulates by fibrous tissue ventrally with the 

 suboperculum. The upper anterior edge of the oper- 

 culum is laterally expanded into a stout facet for fibrous 

 tissue articulation with the concave surface of the up- 

 raised area on the posterior edge of the hyomandibular. 



Suboperculum. — A relatively flat plate, except for a 

 lateral thickening along its edge of fibrous tissue ar- 

 ticulation with the anterior edge of the operculum. From 

 its anterodorsal region the suboperculum is prolonged 

 anteriorly as a narrow shaft to a level slightly forward of 

 the posterior edge of the preoperculum. The anterior end 

 of this shaftlike process connects by a short ligament 

 with the posterior end of the interoperculum. 



Symplectic. — Large; broadly overlain by both the 

 metapterygoid and quadrate; cartilage filled at its pos- 

 terior end; articulates through cartilage with the an- 

 terior end of the hyomandibular, and by interdigitation 

 dorsally with the metapterygoid and anteriorly with the 

 quadrate. Ventrally the symplectic articulates by fi- 

 brous tissue with the preoperculum. 



Palato-Pterygoid Region. 



Palatine. — Expanded dorsally into a horizontal 

 plate; articulates by interdigitation dorsally with the 

 frontal, ventrally with the ectopterygoid, and postero- 

 dorsally with the parasphenoid. Posteroventrally the 

 palatine broadly overlies and articulates by slight inter- 

 digitation with the mesopterygoid. The anterior edge of 

 the portion of the palatine which overlies the mesoptery- 

 goid is cartilage filled in all but the larger study 

 specimens. Anterodorsally the rounded edge of the 

 palatine articulates by fibrous tissue with the concave ar- 

 ticular facet of the maxillary, while a medially directed 

 prong from the anterior edge of the palatine articulates 

 by fibrous tissue with the posteromedial edge of the pre- 

 maxillary. 



Ectopterygoid. — An almost straight shaft of bone, 

 except for a thin posterodorsally expanded portion that 

 interdigitates with the palatine, mesopterygoid, and 

 metapterygoid; articulates by extensive interdigitation 

 dorsally with the palatine and ventrally with the quad- 

 rate. 



Mesopterygoid. — Somewhat variable in shape, but 

 always broadly overlain by the palatine and metaptery- 

 goid; articulates anterodorsally by slight interdigitation 

 with the palatine, while anteroventrally and posteroven- 

 trally it articulates by more extensive interdigitation 

 with, respectively, the ectopterygoid and metaptery- 



Interoperculum. — A stout rod, with a large ventral 

 flange in about the middle of its length; articulates by 

 ligaments anteriorly with the angular in the lower jaw 

 and posteriorly with the anterior end of the suboper- 

 culum. The medial surface of the ventral flange of the in- 

 teroperculum articulates by fibrous tissue with the 

 lateral surface of the epihyal and the posterolateral sur- 

 face of the ceratohyal. 



Preoperculum. — Large; expanded posteroven- 

 trally; its dorsal edge somewhat laterally expanded to 

 form a broad surface of fibrous tissue articulation with 

 the ventral edges of the quadrate, symplectic, and 

 hyomandibular. 



Upper Jaw. 



Premaxillary. — The two premaxillaries are indis- 

 tinguishable fused in the midline and together with the 

 fused teeth form a massive crushing plate; anterior edge 

 of premaxillary forming the border of the upper jaw for 

 about the dorsal two-thirds of that border, ventral to 

 which the maxillary forms the anterior edge of the upper 

 jaw. In the middorsal line the fused premaxillaries form a 

 short posteromedial process which articulates by fibrous 

 tissue with the palatines and the ethmoid-vomer. 

 Posterolaterally the premaxillaries are broadly overlain 

 by and extensively interdigitated with the maxillaries. 

 The fused teeth of the premaxillary are closely similar to 

 those described for Triodon. They are small, discrete, in- 

 dividual pieces formed in the relatively small pulp cavity 

 of the premaxillaries. They move forward toward the 

 edge of the jaw and become closely packed against one 

 another in the bony matrix that surrounds them. By the 

 time they reach the leading edge of the jaw they are so 

 densely packed together that they have lost much of their 

 individuality, at least in large specimens. Whether the 

 teeth at the edge of the jaw ever truly fuse indistinguish- 

 ably with themselves and the premaxillary is again a 



