from its posteroventral region; slightly cartilage filled 

 along its posterior edge; articulates by fibrous tissue 

 anterodorsally with the ectopterygoid and ventrally with 

 the preoperculum. Posteriorly the quadrate articulates 

 through cartilage with the metapterygoid. The inner sur- 

 face of the posteroventral portion of the quadrate that 

 overlies the symplectic forms a concave groove in which 

 the symplectic is held by fibrous tissue. The articulation 

 of the knoblike anterior end of the quadrate with the ar- 

 ticular in the lower jaw is strengthened by the fibrous tis- 

 sue attachment of an anteromedial projection from the 

 anterior edge of the quadrate with the posteromedial 

 edge of the articular. 



Metapterygoid. — Broad anteriorly, but tapering to 

 a point posteriorly; cartilage filled along its anterior and 

 anteroventral edges; articulates through cartilage ante- 

 riorly with the quadrate and by fibrous tissue anteroven- 

 trally with the symplectic and posteroventrally with the 

 hyomandibular. The cartilaginous region between the 

 hyomandibular, metapterygoid, and symplectic is the 

 place of fibrous tissue articulation of the dorsal end of the 

 interhyal. The anterodorsal edge of the metapterygoid 

 articulates by fibrous tissue with the posterior end of the 

 mesopterygoid. 



Symplectic. — A long rod, slightly wider posteriorly 

 than anteriorly; cartilage filled at its posterior and ante- 

 rior edges; articulates by fibrous tissue dorsally with the 

 metapterygoid and anteroventrally with the concavity on 

 the inner surface of the quadrate. 



Palato-Pterygoid Region. 



Palatine. — Wide anteriorly, but becoming a flat 

 vertical plate posteriorly; articulates by fibrous tissue 

 anteromedially with the ethmoid and vomer, postero- 

 medially with the parasphenoid, ventrally with the ec- 

 topterygoid and mesopterygoid, and posterodorsally with 

 the prefrontal. The anterodorsal edge of the palatine, 

 which is produced medially as a slender prong, articu- 

 lates by fibrous tissue with the posterodorsal edge of the 

 maxillary. 



Ectopterygoid. — Only slightly V-shaped, with the 

 apex directed posteriorly; articulates by fibrous tissue 

 dorsally with the slightly overlying palatine, ventrally 

 with the quadrate, and posteriorly with the mesoptery- 

 goid, which it slightly overlies. 



Mesopterygoid. — More or less squarish; articu- 

 lates by fibrous tissue anteriorly with the slightly overly- 

 ing palatine and ventrally with the cartilaginous region 

 between the metapterygoid and quadrate. 



Opercular Region. 



Operculum. — A straight shaft of bone, becoming 

 laterally compressed posteriorly; articulates at its con- 

 cave anterior edge by fibrous tissue with the concave 



facet on the posterior edge of the hyomandibular, while 

 ventrally it overlies and articulates by fibrous tissue with 

 the suboperculum. 



Suboperculum. — Lx)ng and flat; widest in about the 

 middle of its length and becoming much thinner both 

 anteriorly and posteriorly; articulates by fibrous tissue 

 posteriorly with the overlying operculum and anteriorly 

 with the long ligament in which the interoperculum is 

 embedded. 



Interoperculum. — A long delicate needle of bone 

 entirely embedded within the ligament which runs 

 between the angular in the lower jaw and the anterior 

 end of the suboperculum. At the level of the posterior end 

 of the interoperculum, the ligament is particularly firm- 

 ly held to the general connective tissue around the lat- 

 eral surface of the epihyal. 



Preoperculum. — More or less flat throughout its 

 length, except along its dorsal edge where it is slightly ex- 

 panded to form a broadened surface for fibrous tissue ar- 

 ticulation posteriorly with the hyomandibular and ante- 

 riorly with the quadrate. In about the middle of its length 

 the dorsal edge of the preoperculum articulates by 

 fibrous tissue with the cartilaginous region between the 

 hyomandibular, metapterygoid, and symplectic. 



Upper Jaw. 



Premaxillary. — The two premaxillaries ars indis- 

 tinguishably fused in the midline and together with the 

 fused teeth form a large nibbling plate; anterior edge of 

 premaxillary forming about the dorsal two-thirds of the 

 border of the upper jaw, the lower third of the border be- 

 ing formed by the anterior edge of the maxillary. The 

 premaxillaries articulate laterally by fibrous tissue with 

 the broadly overlying maxillaries. Dorsally, however, the 

 expanded upper ends of the maxillaries fit into deep con- 

 cavities in the surface of the premaxillaries and are held 

 there by fibrous tissue. The premaxillaries thus broadly 

 overlie the dorsal ends of the maxillaries. The posterior 

 edge of the fused premaxillaries is overlain by the maxil- 

 laries along all of its length, except for a very short dis- 

 tance medially. The pulp cavity of the fused premaxil- 

 laries is small, shallow, and confined to the dorsal region 

 of the bone. The dorsal roof over the pulp cavity is thin 

 and smooth internally, but the ventral floor of the cavity 

 is thick and highly irregular. The pulp material extends 

 into and between the innumerable bony lamellae and 

 spikes rising from the floor of the eavity. No discrete 

 teeth or dental units of any kind are visible there in 

 either of the two cleared and stained study specimens, or 

 in the two additional sets of much larger jaws, nor can 

 any such structures be distinguished in the hard sub- 

 stance of the biting edge of the fused premaxillaries. A 

 large trituration plate is present on the medial third of 

 the under surface of the fused premaxillaries. In the two 

 relatively small cleared and stained specimens, the tri- 

 turation plate is distinctly divided into closely apposed 



