the pterosphenoid, anterolaterally with the sphenotic, 

 posterolaterally with the pterotic, posteriorly with the 

 exoccipital, and posteromedially with the basioccipital. 

 Anteromedially the prootic articulates for most of its 

 length through cartilage with the parasphenoid, but at 

 its extreme anteromedial end the articulation of the 

 prootic with the short dorsal wing of the parasphenoid is 

 by slight interdigitation. It is at this place of interdigita- 

 tion between the prootic and parasphenoid that the first 

 pharyngobranchial makes a firm ligamentous connec- 

 tion with the cranium. Along its ventral surface the an- 

 terior half of the lateral edge of the prootic helps support 

 the dorsal head of the hyomandibular. The lateral walls 

 of the myodome are formed by the ventromedial sur- 

 faces of the prootics, while the dorsal walls of the myo- 

 dome are formed by medially directed horizontal shelves, 

 which are attached to one another medially by fibrous 

 tissue, from the ventromedial surfaces of the two 

 prootics. The prootics also form most of the anterior wall 

 of the myodome. 



edges of the prootics and thus form the lower part of the 

 anterior edge of the myodome. Behind this restricted 

 area of interdigitation with the anteroventral edge of the 

 prootic, the lateral edges of the parasphenoid articulate 

 through cartilage with the medial edges of the prootics 

 and thus form the floor of the myodome. Anteriorly the 

 parasphenoid broadly overlies and interdigitates with 

 the dorsal surface of the posterior shaftlike portion of the 

 vomer, and, to a much lesser extent, with the ventral sur- 

 face of the latter. The anterior end of the parasphenoid 

 thus essentially possesses a deep concavity into which 

 the shaft of the vomer fits. Anterodorsally the para- 

 sphenoid is tightly held to the ethmoid cartilage, and 

 hence to the ethmoid and prefrontals. 



Pterosphenoid. —Cartilage filled along all of its 

 edges, except medially; articulates through cartilage 

 ventrally with the prootic and ventrolaterally with the 

 sphenotic. Dorsolaterally the pterosphenoid is held by 

 fibrous tissue to the overlying frontal. 



Orbital Region. 



Ethmoid Region. 



Frontal. — Wide posteriorly but tapering to a point 

 anteriorly; articulates posterodorsally and posteroven- 

 trally by overlying and slightly interdigitating with, 

 respectively, the supraoccipital and sphenotic. Oc- 

 casionally the most posterior portion of the frontal slight- 

 ly overlies the anterodorsal portion of the epiotic. 

 Posteromedially on its ventral surface the frontal some- 

 what overlies and articulates by fibrous tissue with the 

 pterosphenoid. Anteriorly the frontal overlies the eth- 

 moid cartilage and articulates by fibrous tissue with the 

 ethmoid and prefrontal. Ventrally the medial edges of 

 the frontals are widely separated by a large cartilaginous 

 mass which is continuous anteriorly with the ethmoid 

 cartilage. 



Prefrontal. — In the form of a basally expanded 

 column; cartilage filled along its dorsomedial and ventro- 

 medial edges; articulates through cartilage along all of its 

 medial surface with the ethmoid and ethmoid cartilage; 

 articulates by fibrous tissue dorsally with the frontal and 

 anteroventrally with the posterior end of the palatine. 

 The prefrontal articulates ventrally with the dorsal sur- 

 faces of the vomer and parasphenoid, by way of the fact 

 that the ethmoid cartilage is firmly attached to the dor- 

 sal surfaces of the posterior region of the vomer and the 

 anterior region of the parasphenoid. 



Parasphenoid. — An elongate shaft with a slight 

 keel along its ventral surface in the region of the orbit. A 

 deep, anteriorly directed cleft is present in the wide pos- 

 terior portion of the parasphenoid, giving rise to the fork- 

 ed region which broadly overlies and interdigitates with 

 the basioccipital and at the same time covers over the 

 medial groove on the ventral surface of the basioccipital 

 leading into the rear of the myodome. About two-thirds 

 the way back, the parasphenoid possesses paired dorso- 

 lateral wings which interdigitate with the anteroventral 



Ethmoid. — The ethmoid remains largely 

 cartilaginous, the ossification being restricted to the sur- 

 face regions. It articulates through cartilage dorsally with 

 the slightly overlying frontals, posterolaterally with the 

 prefrontals, and ventrally with the vomer and para- 

 sphenoid. 



Vomer. — Large and rounded anteriorly, but 

 tapering to a stout shaft posteriorly which interdigitates 

 with the concavity at the anterior end of the para- 

 sphenoid. Dorsally the vomer is attached by fibrous tis- 

 sue to the base of the ethmoid cartilage and thus second- 

 arily with the bases of the ethmoid and prefrontals. At 

 the lateral edges of its rounded anterior portion the 

 vomer articulates by tough fibrous tissue with the medial 

 surfaces of the palatines. 



Mandibular Region. 



Hyomandibular. — Somewhat expanded dorsally, 

 tapering to a stout shaft anteroventrally; cartilage filled 



at its dorsal and anteroventral edges; articulates by fi- 

 brous tissue at its dorsal head with the pterotic pos- 

 teriorly, the sphenotic anterolaterally, and the prootic 

 anteromedially. Along the lower three-fourths of its pos- 

 terior edge it articulates by fibrous tissue with the pre- 

 operculum. Just above the dorsal end of the preoper- 

 culum, the hyomandibular articulates by fibrous tissue 

 at a small groove on its posterior edge with the knob at 

 the dorsal end of the operculum. Anteriorly the hyoman- 

 dibular ends at the cartilaginous plate that lies between 

 it and the symplectic and metapterygoid. 



Quadrate. — Wide posteriorly, tapering to a knob 

 anteriorly for articulation through fibrous tissue with the 

 articular in the lower jaw; cartilage filled along its pos- 

 terior edge; with a long posteriorly directed process from 



