Interoperculum. — A short rod of somewhat ir- 

 regular shape but usually slightly wider in the middle 

 region than at either end; extends from the level of the 

 anterior end of the preoperculum to the level of the pos- 

 terior end of the epihyal; connects by a strong ligament 

 anteriorly to the angular in the lower jaw, while pos- 

 teriorly it connects by a short ligament to the epihyal and 

 by a longer more diffuse ligament to the anterior edge of 

 the suboperculum and, to a lesser extent, of the oper- 

 culum. 



Preoperculum. — Relatively short; moderately ex- 

 panded in the middle region and tapering to narrow ends 

 anteriorly and posteriorly; articulates by fibrous tissue 

 anteriorly along its dorsal edge with the quadrate, while 

 posteriorly the narrow shaft of the preoperculum is held 

 by fibrous tissue along the ventral edge of the hyoman- 

 'dibular and, more dorsally, in the shallow groove on the 

 lateral surface of the hyomandibular. The middle of the 

 dorsal edge of the preoperculum also connects to the fi- 

 brous tissue sheet between the symplectic, hyoman- 

 dibular, and metapterygoid. 



the vomer and ethmoid, and ventromedially with the 

 dorsolateral surface of the dentsu^. 



Lower Jaw. 



Dentary. —Wider posteriorly than anteriorly; its 

 posteromedial surface concave dorsally to accommodate 

 the articular, with which it interdigitates. Just below the 

 articular, the dentary interdigitates with the angular. 

 The ventromedial edges of the two dentaries are held 

 closely together by fibrous tissue. Laterally from its 

 posterodorsal region the dentary articulates by fibrous 

 tissue with the medial surface of the maxillary. Each 

 dentary bears four teeth, like those of the upper jaw, in 

 deep grooves on its outer surface, each of the tooth bear- 

 ing grooves ending posteriorly in a deep socket in which 

 new teeth develop. The sockets are in communication 

 with the large pulp cavity that fills most of the hollow in- 

 terior of the dentary. The pulp cavity communicates 

 with the exterior not only at the sockets but also at its 

 posterior concave region of articulation with the artic- 

 ular. 



Upper Jaw. 



Premaxillary. — A slightly curved plate, wider dor- 

 sally than ventrally; its posterodorsal region flattened for 

 articulation by fibrous tissue with the anterior edges of 

 the vomer and ethmoid; the anterior edge of the upper 

 jaw formed by the premaxillary, except for a short dis- 

 tance ventrally where it is formed by the maxillary; the 

 dorsomedial edges of the two premaxillaries held in close 

 apposition by fibrous tissue. Each premaxillary bears 

 four teeth in a single row in the two specimens examin- 

 ed. The teeth are borne in relatively deep and elongate 

 grooves on the outer surface of the premaxillary. The 

 lowermost teeth are shaftlike and taper to sharp points, 

 but more dorsally they are flattened and blunter at the 

 distal ends, except when they first erupt through the gum 

 as replacement teeth, at which time they too are sharp 

 pointed but apparently rapidly become worn down to 

 bluntness through use. At the posterior end of each tooth 

 bearing groove there is a deep socket in which new teeth 

 develop before moving forward to replace the old ones. 

 Most of the interior of the premaxillary contains the den- 

 tal pulp from which the new teeth develop. This pulp 

 cavity communicates with the exterior not only by the 

 deep sockets in which the new teeth develop but also by a 

 large hole in the posterodorsal surface of the maxillary. 

 The premaxillary articulates by extensive inter- 

 digitation with the maxillary along all of its posterior 

 edge, except for a short distance dorsally. 



Maxillary. — Widest ventrally, with a deep in- 

 dentation along its lower posterior edge; articulates by 

 extensive interdigitation anteriorly along all of its length 

 except extremely ventrally with the premaxillary, which 

 it somewhat overlies. The maxillary articulates by fi- 

 brous tissue posterodorsally with the anterior edges of 



Articular. — Small; its posterior edge with a con- 

 cavity for articulation by fibrous tissue with the anterior 

 knoblike process of the quadrate. Anteriorly the articular 

 interdigitates with the concave upper half of the postero- 

 medial surface of the dentary. On the medial side of its 

 ventral edge the articular interdigitates with the angular, 

 but the lateral surfaces of the two bones are not in con- 

 tact; rather, they are slightly separated by the dentary. 

 The sesamoid articular is a flattened nubbin of bone 

 mostly held to the medial surface of the anterior end of 

 the articular just behind the region where the upper 

 medial edge of the articular meets the medial surface of 

 the dentary. 



Angular. — Small and mainly confined to the 

 medial surface of the lower jaw; articulates by inter- 

 digitation dorsomedially with the articular, while dorso- 

 laterally, anteriorly, and ventrally it interdigitates with 

 the dentary. Posteriorly the angular connects by liga- 

 ment with the anterior end of the interoperculum. 



BRANCHIAL APPARATUS. 



Hyoid Arch, Branchiostegal Rays, and Urohyal. 



Hypohyals. — Both elements well developed; ven- 

 tral element larger than the dorsal element; dorsal ele- 

 ment cartilage filled at its ventral and posterior edges, 

 the ventral element cartilage filled at its dorsal and pos- 

 terior edges. The two elements articulate with one 

 another and with the ceratohyal mainly through car- 

 tilage, but on their medial surfaces the two hypohyals 

 have thin extensions which interdigitate with one 

 another. The anteromedial edges of both elements ar- 

 ticulate by fibrous tissue with their opposite members. 

 The dorsomedial edge of the dorsal hypohyal articulates 



