Opercular Region. 



Operculum. — Thin and expanded ventrally; a 

 dorsally directed process for muscle attachment present 

 above its articular region with the preoperculum; articu- 

 lates ventrally by fibrous tissue with the suboperculum, 

 which it broadly overlies, while along its upper anterior 

 edge the operculum is slightly expanded laterally into 

 the articular facet whose concave face attaches by fi- 

 brous tissue to the preoperculum. 



Suboperculum. — Thin and delicate in its regions 

 that lie behind and below the operculum, but slightly 

 thicker at its anterodorsal end; articulates by fibrous tis- 

 sue dorsally with the overlying operculum, while at the 

 end of its emterodorsal process it connects by a ligament 

 with the upper prong at the posterior end of the in- 

 teroperculum. 



Interoperculum. — A long sturdy rod for all of its 

 length, except posteriorly where it bifurcates into two 

 thin prongs. The more ventral of these two prongs lies in 

 the fibrous tissue sheet between the branchiostegal rays 

 and the region of the operculum and suboperculum. The 

 dorsal prong connects by ligament with the anterodorsal 

 process of the suboperculum. Anteriorly the interoper- 

 culum connects by ligament with the angular in the 

 lower jaw. Just in front of its ventral prong the medial 

 surface of the interoperculum is held by fibrous tissue to 

 the area of articulation between the epihyal and in- 

 terhyal. 



Preoperculum. — Relatively thin and narrow 

 throughout its length, being only slightly expanded ven- 

 trally in its middle region in the smaller specimen, but 

 about one-third wider in the larger specimen; articu- 

 lates by fibrous tissue along the posterior half of its dor- 

 sal edge with the hyomandibular and along the anterior 

 half of its dorsal edge with the quadrate and the region of 

 the symplectic, metapterygoid, and interhyal. Along the 

 upper portion of its posterior edge the preoperculum 

 possesses a slight convexity for articulation through fi- 

 brous tissue with the operculum. 



Upper Jaw. 



Premaxillary. — Posteromedial arm thick and 

 sturdy; together with the fused teeth forms a massive 

 crushing plate; its anterior edge forming the anterior 

 border of the upper jaw, except for a short distance ven- 

 trally where the maxillary forms the border. The dorso- 

 medial surfaces of the two premaxillaries are firmly held 

 to one another by fibrous tissue and extensive interdigi- 

 tation, which, especially in the larger specimen, takes 

 the form of numerous small projections from the 

 otherwise flattened medial surfaces of apposition of each 

 premaxillary alternating with one another. The postero- 

 lateral surface of the premaxillary is strongly interdigi- 

 tated with the overlying maxillary. The premaxillary 



contains an elongate internal cavity which houses the 

 dental pulp and which communicates with the exterior 

 by two openings on the posterior edge of the interdigi- 

 tated surfaces of the premaxillary and maxillary, as well 

 as at numerous smaller openings along the lateral surface 

 of the tooth bearing region, especially dorsally. The pos- 

 teromedial arm of the premaxillary articulates by fi- 

 brous tissue posteriorly with the vertical anterior face of 

 the ethmoid. The fused teeth of the jaws retain much of 

 their individual identity and are particularly clearly seen 

 at the edge of the jaw. About 25 to 30 of these small den- 

 tal units, with the rounded faces oriented toward the 

 biting edge of the jaw and the apparently slightly con- 

 cave edges oriented toward the pulp cavity, can be seen 

 packed closely together at the edge of the jaw, being held 

 immovably in place by an extremely dense bony matrix. 

 How much fusion actually takes place between the tooth 

 elements themselves and between the tooth elements 

 and the bony matrix of the premaxillary could only be 

 told by a histological examination not attempted here. 

 The tooth elements become less and less distinctly seen 

 at the surface further away from the edge of the jaw, ex- 

 cept at the extreme dorsal edge of the tooth bearing 

 lateral surface, where the primordia of the individual 

 teeth can be seen forming in the small pockets in the 

 matrix that open by pores to the exterior on the lateral 

 surface. The primordia obviously move gradually toward 

 the distal crushing edge of the jaw and become more fully 

 impacted with the matrix of the premaxillary as new 

 primordia form behind it. There are about 10 to 15 such 

 small teeth in various stages of incorporation with the 

 matrix between the generative region at the inner edge of 

 the tooth bearing surface and the crushing outer edge of 

 the jaw. In the medial region of its ventral surface the 

 premaxillary bears a massive trituration plate formed of 

 fused teeth. The process by which this plate is formed on 

 the undersurface of the premaxillary can be seen at the 

 posterior edge of the plate. Four or five anteroposterior^ 

 compressed teeth are formed in deep sockets at the pos- 

 terior edge of the plate and are clearly evident as entirely 

 individual and separate units held relatively loosely in 

 their sockets. Immediately in front of this row of newly 

 formed teeth in their individual sockets, the surface of 

 the trituration plate no longer shows individual tooth 

 elements, and only the ridges and grooves on the surface 

 give evidence of what are evidently the now fused teeth 

 that had moved up from the formation area. The medial 

 edges of the trituration plates of either premaxillary do 

 not meet in the midline but, rather, are separated by a 

 slight gap. 



Maxillary. — Somewhat curved forward at both 

 ends; forms the anterior border of the upper jaw at its 

 ventral end. Posteromedially the maxillary possesses a 

 relatively long, but thin, process which overlies the 

 lateral and most of the ventral edges of the postero- 

 medial arm of the premaxillary. The lateral surface of 

 this posteromedial arm of the maxillary is slightly con- 

 cave, forming the articular surface which makes fibrous 

 tissue contact with the medial surface of the anterodor- 



