tales with the anterior end of the basioccipital, while just 

 anterior to this region the parasphenoid interdigitates 

 with the ventromedial edges of the prootics. About three- 

 fourths the way back its length the parasphenoid gives 

 rise to its dorsolateral wings which interdigitate with the 

 anteromedial edges of the prootics. 



Pterosphenoid. — A small, thin plate; cartilage 

 filled at its dorsal and lateral edges; articulates by slight 

 interdigitation dorsally with the frontal, laterally with 

 the sphenotic and ventrally with the prootic. 



Ethmoid Region. 



Ethmoid. — More or less T-shaped in cross section; 

 its dorsal surface laterally expanded, increasingly so pos- 

 teriorly, while its ventral surface is drawn out into a thin 

 medial flange in the posterior two-thirds of its length; ar- 

 ticulates by slight interdigitation pwsteriorly with the 

 frontals and prefrontals, posteroventrally with the some- 

 what overlying parasphenoid and anteroventrally with 

 the vomer. 



Vomer. — Thin and laterally compressed pos- 

 teriorly, but dorsoventrally depressed anteriorly; anterior 

 end concave; anterolateral edge with a deep indentation 

 around which the anterodorsal forked region of the pala- 

 tine fits and is held by fibrous tissue; articulates by slight 

 interdigitation dorsally with the ethmoid and pos- 

 teriorly with the concave anterior end of the 



Mandibular Region. 



Hyomandibular. — Expanded dorsally, tapering 

 gradually to a stout shaft anteroventrally; cartilage filled 

 at its dorsal and anteroventral edges; articulates by 

 fibrous tissue dorsolaterally with the sphenotic and 

 pterotic and dorsomedially with the prootic and pterotic. 

 The ventral three-fourths of the posterior edge of the hyo- 

 mandibular articulates by fibrous tissue with the 

 preoperculum, while just above the dorsal end of the 

 latter the hyomandibular supports the articular face of 

 the operculum. At its anteroventral end the hyo- 

 mandibular makes fibrous tissue contact mainly with the 

 preoperculum, and only secondarily with the symplectic 

 and metapterygoid. 



Quadrate. — Somewhat wider posteriorly than 

 anteriorly; prolonged posteroventrally into a thin taper- 

 ing piece below the metapterygoid; cartilage filled only 

 for a short distance along its posterodorsal edge; articu- 

 lates by fibrous tissue anteriorly with the articulfir in the 

 lower jaw and ventrally with the preoperculum. It ar- 

 ticulates by interdigitation dorsally with the overlying 

 ectopterygoid, while posterodorsally it overlies and inter- 

 digitates with the metapterygoid. 



Metapterygoid. — A more or less rounded plate; 

 cartilage filled only for a short distance along its antero- 



ventral edge; articulates by interdigitation anteriorly 

 with the ectopterygoid and palatine, dorsally with the 

 mesopterygoid, anteroventrally with the quadrate, and 

 posteroventrally with the symplectic. 



Symplectic. — A thin delicate rod; cartilage filled at 

 its posterior edge; articulates by interdigitation dorsally 

 with the metapterygoid and by fibrous tissue ventrally 

 with the preoperculum. 



Palato-Pterygoid Region. 



Palatine. —Expanded posteriorly; a short 

 anteriorly directed cleft present in the anterior part of its 

 dorsal edge; articulates anterodorsally by firm fibrous 

 tissue attachment of its cleft surface in the anterolateral 

 indentation of the vomer; articulates by interdigitation 

 ventrally with the ectopterygoid, posteroventrally with 

 the metapterygoid, and posterodorsally with the 

 mesopterygoid. 



Ectopterygoid. — Roughly triangular, its posterior 

 end tapering to a blunt point; articulates by interdigi- 

 tation dorsally with the palatine, posteriorly with the 

 metapterygoid, and ventrally with the quadrate, which it 

 broadly overlies. 



Mesopterygoid. — Broad anteriorly, but becoming 

 narrower posteriorly; articulates by interdigitation 

 anteriorly with the palatine and ventrally with the 

 metapterygoid, both of which bones overlie the 

 mesopterygoid to a variable extent. 



Opercular Region. 



Operculum. — Elongate; wide in the middle of its 

 length, but tapering gradually to points anteriorly and 

 posteriorly; its dorsolateral surface produced into a 

 strong keel which extends dorsally as a stout shaft above 

 the articular facet for connection with the hyomandibu- 

 lar. This dorsal process serves as a place of attachment 

 for the muscles originating on the posterolateral surface 

 of the frontal. The operculum articulates by fibrous 

 tissue anterodorsally with the upper posterior edge of the 

 hyomandibular and ventrally with the suboperculum, 

 which it overlies. In about the middle of its anterior edge 

 the operculum makes ligamentous contact with the rod- 

 like posterior end of the interoperculum. 



Suboperculum. — Thin and delicate; rounded 

 ventrally but produced dorsally into two processes, the 

 more anterior of which is by far the longer. The posterior 

 edge of the more anterior of these two dorsal processes ar- 

 ticulates by fibrous tissue with the anterior edge of the 

 operculum. The lateral surface of the rounded ventral 

 portion of the suboperculum is broadly overlain by the 

 ventral end of the operculum. 



Interoperculum. — A long rod, with a ventral flange 

 along the middle region of its ventral edge; articulates by 



