Sphenotic. — Relatively small and confined to the 

 posterior wall of the orbit; cartilage filled along all of its 

 edges of articulation with the other cranial bones; ar- 

 ticulates by interdigitation anterodorsally with the fron- 

 tal, posterodorsally with the pterotic, anteroventrally 

 with the pterosphenoid, and ventrally with the prootic. 

 The ventral edge of the sphenotic is deeply concave for 

 articulation by fibrous tissue with the anterolateral 

 region of the dorsal head of the hyomandibular. This con- 

 cavity of the sphenotic is continuous with that along the 

 lateral edge of the prootic, which supports the antero- 

 medial region of the head of the hyomandibular, and with 

 that of the pterotic forming the main support for that 

 head. 



Epiotic. — Relatively large, but thin; cartilage filled 

 along its medial edge; articulates by interdigitation 

 anteriorly with the frontal, medially with the exoccipital 

 and posteriorly with the pterotic. 



Prootic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones, except anteri- 

 orly; forms all of the walls of the myodome, except for the 

 medial portion of the ventral wall and the short section of 

 the anterior wall formed by the parasphenoid and for the 

 extreme posterior portion of the wall formed by the 

 basioccipital; articulates by interdigitation anteriorly 

 with the dorsolateral wing of the parasphenoid, dorso- 

 medially with the pterosphenoid, dorsolaterally with the 

 sphenotic, ventrolaterally with the pterotic, ventro- 

 medially with the parasphenoid and basioccipital, and 

 posteriorly with the exoccipital. From its oblique medial 

 surface the prootic gives rise to the horizontal plate of 

 bone which meets its opposite member in the midline to 

 form the dorsal roof of the myodome. These two projec- 

 tions interdigitate with one another in the midline and 

 each possesses a small foramen laterally near its dorsal 

 edge. Another pair of foramina are present in the ventral 

 wall of the myodome to either side of the midline, where 

 there is a slight gap in the interdigitated surfaces of the 

 prootics and the parasphenoid. Anteromedially from the 

 dorsal roof of the myodome the prootics support by fi- 

 brous tissue the posterior end of the basisphenoid. At the 

 concavity along its lateral edge the prootic articulates by 

 fibrous tissue with the anteromedial region of the dorsal 

 head of the hyomandibular. 



Orbital Region. 



Frontal. — Large and massive; laterally expanded 

 posteriorly but tapering to a bluntly rounded end ante- 

 riorly. Throughout its entire length the medial edge of 

 the frontal is broadly, but not deeply, concave and covers 

 a core of cartilage that is continuous anteriorly with the 

 cartilage of the ethmoid region and posteriorly with that 

 of the occipital region. The frontal articulates by inter- 

 digitation dorsomedially in the posterior half of its length 

 with its opposite member, while directly postero- 

 medially it interdigitates with the supraoccipital. 

 Posterolaterally on its dorsal surface it interdigitates 



with the exoccipital and epiotic. Posteroventrally it in- 

 terdigitates with, from anterior to posterior, the ptero- 

 sphenoid, sphenotic, and pterotic. The ventromedial 

 edges of the two frontals are in close fibrous tissue con- 

 tact for most of their lengths, diverging from one another 

 only posteroventrally just in front of the pterosphenoids 

 and anteriorly just behind the prefrontals. Anteriorly the 

 frontal broadly overlies and slightly interdigitates with 

 the ethmoid, while anterolaterally it interdigitates with 

 the dorsomedial edge of the prefrontal. 



Prefrontal. — Relatively large, both dorsally and 

 ventrally; cartilage filled along all of its ventromedial 

 edges where it is continuous with the ethmoid cartilage; 

 articulates dorsally by interdigitation with the frontal and 

 the posterolateral edges of the ethmoid, while postero- 

 ventrally it articulates through cartilage and fibrous tis- 

 sue with the anterodorsal surface of the parasphenoid. 

 Along most of its ventral edge the prefrontal articulates 

 through cartilage and interdigitation with the long pos- 

 terior portion of the palatine. In about the middle of its 

 medial surface the prefrontal is deeply concave, so that a 

 horizontal canal is formed between the prefrontal and 

 the ethmoid cartilage, with which the prefrontal is 

 otherwise continuous. The nerves and blood vessels that 

 course through this canal run not only to the olfactory 

 organ but also to the anteriormost regions of the head by 

 way of a similar canal between the palatine and ethmoid. 



Parasphenoid. — Elongate, with a thin ventral keel 

 throughout most of its length. The anterior end of the 

 parasphenoid is deeply concave to accommodate the 

 posterior end of the vomer, with which it is interdigi- 

 tated. The parasphenoid also interdigitates antero- 

 laterally with the posterior portion of the palatine, while 

 anterodorsally it articulates through cartilage and fi- 

 brous tissue with the prefrontal. Posteriorly the para- 

 sphenoid somewhat overlies and strongly interdigitates 

 with the basioccipital, while at the same time leaving 

 open the small canal in the midline between their 

 otherwise interdigitated surfaces. About five-sixths of 

 the way back in its length, the parasphenoid gives rise to 

 its paired dorsolateral wings which interdigitate with the 

 anteromedial edges of the prootics and form a section of 

 the anterior wall of the myodome. On its ventral surface 

 between the level of its dorsolateral wings and its 

 meeting with the basioccipital, the lateral edges of the 

 parasphenoid interdigitate with the ventromedial edges 

 of the prootics, and in so doing the parasphenoid forms 

 the medial portion of the ventral wall of the myodome. 

 At the posterior end of its ventral keel the parasphenoid 

 becomes thickened and posterolaterally expanded into a 

 bony knob to which the distal end of the first (suspen- 

 sory) pharyngobranchial and associated muscles are at- 

 tached. 



Pterosphenoid. — A thick plate; broadly cartilage 

 filled along its dorsal and lateral edges; articulates by in- 

 terdigitation dorsally with the frontal, posterolaterally 

 with the sphenotic, and posteroventrally with the prootic. 



