tates with the mesopterygoid. Anterodorsally the pala- 

 tine is expanded laterally to form the articular facet for 

 support of the maxillary through fibrous tissue. 



Ectopterygoid. — Somewhat V-shaped, with the 

 apex directed posteriorly; articulates by interdigitation 

 dorsally with the palatine, posteriorly with the 

 mesopterygoid, which it broadly overlies, and ventrally 

 with the quadrate, which slightly overlies it. 



Mesopterygoid. — Variable in shape, but usually 

 thin anteriorly and thickened at its posterior edge. It ap- 

 pears to be relatively small as seen laterally, because it is 

 overlain by each of the four bones with which it articu- 

 lates by interdigitation; anterodorsally with the pala- 

 tine, anteriorly with the ectopterygoid, anteroventrally 

 with the quadrate, and posteroventrally with the 

 metapterygoid. 



Opercular Region. 



Operculum. — Thin and expanded posteroven- 

 trally; a dorsally directed process from its dorsal sur- 

 face present for muscle attachment; articulates by 

 fibrous tissue dorsally at its flattened articular surface 

 with the upper posterior edge of the hyomandibular, 

 while ventrally it articulates with the deeply cleft region 

 of the suboperculum, which it slightly overlies. In about 

 the middle of its anterior edge the operculum articulates 

 by a tough ligament with the rodlike interoperculum. 



Suboperculum. — Thin; deeply and broadly cleft in 

 its upper region; the portion of the bone anterior to the 

 cleft somewhat prolonged dorsally; articulates by fibrous 

 tissue along the edges of its cleft region with the slightly 

 overlying operculum. 



Interoperculum. — A long rod, with a small ventral 

 flange from about the middle of its ventral edge; articu- 

 lates by strong ligaments posteriorly with the operculum 

 and anteriorly with the angular in the lower jaw. The 

 ventral flange articulates by fibrous tissue with the 

 lateral surface of the epihyal. 



Preoperculum. — Large; expanded posteroventral- 

 ly; slightly convex laterally; the anterior half of its dorsal 

 edge somewhat laterally expanded to present a broad 

 surface for fibrous tissue articulation with the ventral 

 edge of the quadrate; articulates by fibrous tissue along 

 the posterior half of its dorsal edge with the quadrate, 

 symplectic, interhyal, and metapterygoid. 



Upper Jaw. 



Premaxillary. — Posteromedial arm short; 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 ventrally where the max- 

 illary forms the border. The premaxillaries articulate 

 dorsomedially with one another by fibrous tissue, with 



the articulation strengthened by a single row of a dozen 

 or more well-developed medial projections from the dor- 

 somedial surface of each premaxillary. These projec- 

 tions alternate with one another and decrease in size 

 anteriorly. The short posteromedial arm of the premaxil- 

 lary articulates by fibrous tissue with the anteroventral 

 surface of the vomer. Posteriorly and laterally the 

 premaxillary is firmly articulated by extensive interdigi- 

 tation with the broadly overlying maxillary. The 

 premaxillary contains an internal cavity which commu- 

 nicates with the exterior through a small opening placed 

 just lateral to the posteromedial arm that articulates 

 with the vomer. This cavity in the premaxillary contains 

 the dental pulp, which continually gives rise to the long, 

 thin, rodlike, highly modified teeth. These teeth lie 

 parallel to the anterior edge of the premaxillary and are 

 constantly being moved forward to replace those dental 

 lamellae worn down through use. There are usually 5 to 

 10 of these dental lamellae present in each premaxillary, 

 of which only the most anterior one or two are actually 

 exposed at the edge of the jaw. However, the lamellae 

 posterior to them can be seen easily through the very thin 

 portion of the premaxillary covering them, giving the im- 

 pression that many more of the dental lamellae are ex- 

 posed than is actually the case. On its ventral surface the 

 premaxillary bears a variable number (two to six) of 

 more or less anteroposteriorly compressed blunt teeth, 

 set in shallow sockets, in a longitudinal row just lateral to 

 its medial edge. These teeth, like the dental lamellae, are 

 replaced from the posterior end of the series by the ac- 

 tivity of the pulp tissue. 



Maxillar>'. — Curved forward and slightly expanded 

 dorsally and ventrally; broadly overlies and firmly inter- 

 digitates with the premaxillary along all of its length, ex- 

 cept for a short distance ventrally where it is free of the 

 premaxillary and forms the anterior border of the upper 

 jaw. Posterodorsally at a slight groove on its surface the 

 maxillary is supported through fibrous tissue by the 

 anterodorsal edge of the palatine. The ventromedial sur- 

 face of the maxillary articulates by fibrous tissue with 

 the dorsolateral surface of the dentary. 



Lower Jaw. 



Dentary. — Somewhat squarish; articulates poste- 

 riorly by fibrous tissue and interdigitation with the artic- 

 ular, which it broadly overlies laterally but only slightly 

 overlies medially. Posteroventrally the dentary articu- 

 lates by fibrous tissue, or in large specimens by inter- 

 digitation, with the angular. The dorsolateral surface of 

 the dentary articulates by fibrous tissue with the ventro- 

 medial surface of the maxillary. The dentary is hollow in- 

 ternally, although not as extensively so as the premaxil- 

 lary. The cavity contains the dental lamellae producing 

 pulp material, which functions in the same way as de- 

 scribed for the premaxillary. The pulp cavity opens to 

 the exterior at the region of articulation of the anterior 

 end of the articular with the dentary. Only long, thin, 

 rodlike, dental lamellae are produced, there being no 



