fibrous tissue with the middle of the ventral edge of the 

 palatine; articulates by extensive interdigitation ventral- 

 ly with the quadrate and posteriorly with the 

 mesopterygoid. 



Mesopterygoid. — A small plate extensively inter- 

 digitated anteriorly with the ectopterygoid and, to a 

 slight extent, with the quadrate in some specimens, and 

 posteriorly with the metapterygoid; its anteroventral 

 edge in contact with the cartilaginous region between the 

 quadrate and the mesopterygoid. 



Opercular Region. 



Operculum. — More or less thin and flat, except 

 dorsally where it thickens into a rounded shaft with an 

 articular facet for articulation by fibrous tissue with the 

 posterior edge of the dorsal region of the hyomandibular, 

 while ventrally it overlies and articulates by fibrous 

 tissue with the suboperculum. 



Suboperculum. — Very thin, flat, and oblong; held 

 to the overlying operculum by fibrous tissue. 



Interoperculum. — A straight rod, tapering slightly 

 to a point anteriorly; extends from the region of the in- 

 terhyal to just beyond the anterior end of the preoper- 

 culum; articulates by a tough ligament anteriorly with 

 the angular in the lower jaw, while at its posterior end 

 two ligaments are present. One is short and connects 

 with the dorsal surface of the epihyal just below the ar- 

 ticulation of the latter with the interhyal, while the other 

 one is long and runs posteriorly to connect to the anterior 

 edge of the operculum just above the point where the 

 operculum begins to overlie the suboperculum. 



Preoperculum. — Expanded in its middle region; its 

 dorsal surface only slightly flattened for articulation with 

 the quadrate; articulates along the posterior portion of 

 its dorsal edge by fibrous tissue with the hyomandib- 

 ular, along the middle portion of its dorsal edge through 

 cartilage and fibrous tissue with the symplectic, 

 metapterygoid, and interhyal, and along the anterior 

 portion of its dorsal edge by fibrous tissue with the 

 quadrate. 



Upper Jaw. 



Premaxillary. — A medially curved plate, wider 

 dorsally than ventrally; its posterodorsal edge slightly 

 concave to articulate by fibrous tissue with the anterior 

 ends of the ethmoid and vomer. It also articulates by 

 fibrous tissue laterally with the medial surface of the 

 palatine, although the main articulation of the latter is 

 with the maxillary. The anterior edge of the premaxillary 

 forms the entire anterior border of the upper jaw and only 

 shares the border with the maxillary ventrally. The 

 premaxillary is in close apposition with the maxillary 

 and articulates immovably with it by fibrous tissue and 

 slight interdigitation. The flat dorsomedial edges of the 



two premaxillaries are held closely to one another by 

 fibrous tissue. Each premaxillary bears five teeth, three 

 in an outer row and two in an inner row. Both of these 

 rows are in close contact with one another. The teeth are 

 borne in very shallow depressions on the surface of the 

 premaxillary in their fully formed condition. They 

 develop, however, from large deep sockets which open to 

 the surface just at the base of the old tooth which they 

 will replace. As a tooth erupts to the surface to replace an 

 old tooth, the socket from which it arose apparently is 

 overgrown or filled in with bone and disappears and the 

 tooth rests only in a shallow depression on the surface of 

 the premaxillary. Most of the interior of the premaxillary 

 is given over to the dental pulp cavities, these cavities 

 communicating with the exterior through numerous 

 holes on the inner and outer surfaces of the bone. 



Maxillary. — Widest ventrally, becoming narrow in 

 the middle, and expanded again dorsally where it ar- 

 ticulates with the anterior head of the palatine; articu- 

 lates firmly with the premaxillary by fibrous tissue and 

 slight interdigitation. The medial surfaces of the ventral 

 rti-ioiis of the maxillary and, to a lesser extent, of the 

 premaxillary articulate with the lateral surface of the up- 

 per portion of the dentary, by fibrous tissue. The postero- 

 dorsal surface of the maxillary articulates by fibrous 

 tissue with the lateral expansions of the ethmoid and 

 vomer and with the medial surface of the palatine. 



Dentary.— Wide posteriorly; its posterior edge 

 concave to accommodate the anterior portion of the £tr- 

 ticular, with which it articulates by interdigitation. 

 Posteroventrally the dentary articulates by fibrous tissue 

 and slight interdigitation with the angular. From the 

 lateral surface of its posterodorsal region the dentary ar- 

 ticulates by fibrous tissue with the medial surfaces of the 

 maxillary and, to a lesser extent, of the premaxillary. 

 Ventromedially the dentary has broad denticulations 

 with which it articulates through fibrous tissue with its 

 opposite member. Each dentary bears three teeth in a 

 single row, corresponding to the outer row in the premax- 

 illary. The teeth are borne in shallow sockets on the sur- 

 face of the dentary and are replaced by new teeth 

 developing in deep sockets just as described for the teeth 

 of the upper jaw. Most of the interior of the dentary is 

 filled with the pulp cavity of the developing teeth, the 

 cavity opening to the exterior at the concave posterior 

 edge of the dentary as well as at pores on the inner and 

 outer surface of the bone. 



Articular.— Wide along its posterior edge, with a 

 concave facet for articulation by fibrous tissue with the 

 anterior knob of the quadrate; articulates by interdigi- 

 tation with the concave posteromedial surface of the den- 

 tary. At its extreme posteroventral edge the articular 

 interdigitates slightly with the angular. The sesamoid ar- 

 ticular is a small nubbin of bone closely held by fibrous 



