ligaments posteriorly with the operculum and anteriorly 

 with the angular in the lower jaw. The ventral flange ar- 

 ticulates by fibrous tissue with the lateral surface of the 

 epihyal. 



Preoperculum. — Large, expanded posteroventral- 

 ly; slightly expanded laterally along the anterior half of 

 its dorsal edge for articulation by fibrous tissue with the 

 ventral surface of the quadrate; articulates by fibrous tis- 

 sue posterodorsally with the hyomandibular and antero- 

 dorsally with the symplectic, metapterygoid, and 

 quadrate. 



Upper Jaw. 



Premaxillary. — Posteromedial arm short; together 

 with the fused teeth forms a relatively sharp-edged cut- 

 ting plate; its anterior edge forming the anterior border of 

 the upper jaw, except for a short distance ventrally where 

 the maxillary forms the border. The dorsomedial edges of 

 the two premaxillaries articulate with one another by 

 fibrous tissue, with the articulation strengthened by 15 to 

 20 short but firm projections, which decrease in size 

 anteriorly in the series, from the dorsomedial edge of 

 each premaxillary. The posteromedial arms of the 

 premaxillaries articulate by fibrous tissue with the 

 posteroventral surface of the vomer. Posterolaterally the 

 premaxillary articulates by interdigitation with the 

 overlying maxillary. The premaxillary is relatively hol- 

 low interiorly to accommodate the dental pulp. The long, 

 thin, rodlike teeth produced by the dental pulp lie paral- 

 lel to the edge of the premaxillary and are continuously 

 moved forward to replace those being worn away by use 

 at the biting edge. About 15 to 20 such rodlike dental 

 lamellae are present in each premaxillary. Only the most 

 anterior one or two lamellae are exposed at the cutting 

 edge of the jaw, but the other lamellae are easily seen 

 posterior to them because of the thinness of the overlying 

 anterior region of the premaxillary. The pulp-filled inter- 

 nal cavity of the premaxillary is open to the exterior 

 through a small canal just lateral to the base of its 

 posteromedial arm. 



Maxillary. —Not especially curved forward; widest 

 ventrally where it forms the border of the upper jaw; 

 broadly overlies and firmly interdigitates with the pre- 

 maxillary along all of its length except at its expanded 

 ventral edge, where its medial surface articulates by 

 fibrous tissue with the dorsolateral surface of the den- 

 tary. The maxillary articulates posterodorsally by 

 fibrous tissue at a groove on its surface with the pala- 

 tine. 



Dentary- — Somewhat squarish; its posterior end 

 concave to accommodate and interdigitate with the 

 anterior portion of the articular; the dentary overlies the 

 articular more extensively laterally than medially. 

 Posteroventrally the dentary articulates by fibrous tissue 



or slight interdigitation with the angular, while antero- 

 medially it articulates by fibrous tissue with its opposite 

 member. The connection with its opposite member is 

 strengthened by about 15 stubby medial projections from 

 each dentary which alternate with one another and de- 

 crease in size anteriorly, as is the case with the pre- 

 maxillary. The dorsolateral surface of the dentary at- 

 taches by fibrous tissue to the ventromedial surface of 

 the maxillary. About 15 dental lamellae are present in 

 the dentary, in the same manner as described for those of 

 the premaxillary. The dentary is filled with the pulp 

 cavity that produces these rodlike teeth, and the cavity 

 opens to the exterior at the concave posterior end of the 

 dentary just in front of the anterior end of the articular. 



Articular. — Somewhat triangular in shape; carti- 

 lage filled at its anterior edge; the anterior portion of the 

 articular fits into and interdigitates with the concave 

 posterior end of the dentary. The posterior edge of the ar- 

 ticular is thickened around the concavity at which it ar- 

 ticulates by fibrous tissue with the quadrate. Postero- 

 ventrally the articular articulates by fibrous tissue with 

 the angular, but in larger specimens this articulation is 

 by interdigitation. The sesamoid articular is a thin nub- 

 bin of bone lying alongside the medial surface of the ar- 

 ticular just behind the articulation of the latter with the 

 dentary. 



Angular. — Small; articulates by fibrous tissue, or 

 slight interdigitation in larger specimens, with the den- 

 tary and articular. Posteriorly it articulates by a liga- 

 ment to the anterior end of the interoperculum. 



BRANCHIAL APPARATUS. 



Hyoid Arch and Branchiostegal Rays. 



Urohyal, basihyal, and interhyal— Absent. 



Hypohyal. —Dorsal hypohyal absent; ventral 

 hypohyal wide posteriorly but tapering to a blunt point 

 anteriorly; cartilage filled at its posterior edge; articu- 

 lates through cartilage posteriorly with the ceratohyal 

 and by fibrous tissue medially with its opposite member. 



Ceratohyal. —Elongate and slightly expanded pos- 

 teriorly; cartilage filled at its anterior and posterior 

 edges; articulates anteriorly through cartilage with the 

 hypohyal and posterodorsally by interdigitation with the 

 epihyal. The ceratohyal supports by fibrous tissue all of 

 the six branchiostegal rays. 



Epihyal. — Cartilage filled at its anterior and 

 ventral edges; articulates anteriorly by interdigitation 

 with the ceratohyal. Along its lateral surface the epihyal 

 articulates by fibrous tissue with the ventral flange of the 

 interoperculum, while dorsally it connects, without the 

 intervention of an interhyal, with the symplectic-metap- 

 terygoid region. 



