teriorly with the epiotic, posterolaterally with the 

 sphenotic, anteriorly with the ethmoid, and antero- 

 laterally with the prefrontal. Posteromedially on its ven- 

 tral surface in the rear of the orbit the frontal inter- 

 digitates medially with the pterosphenoid and ventrally 

 with the sphenotic. Medially the two frontals are in close 

 apposition with one another and articulate either 

 through fibrous tissue or interdigitation, but at one or 

 more places along their lengths there is usually a small 

 amount of cartilage visible between the two halves. The 

 frontal interdigitates anteriorly with the ethmoid and 

 anterolaterally with the prefrontal, both of which bones 

 it overlies. 



Prefrontal. — Cartilage filled along most of its 

 broad medial surface except for the long anterior strut 

 that lies alongside the ethmoid. Posteromedially the 

 edges of the two prefrontals interdigitate with one 

 another but anteromedially the prefrontals articulate 

 through cartilage with the ethmoid, except for the long 

 anterior strut which lies alongside and interdigitates 

 with the lateral surface of the anterior half of the eth- 

 moid, the shaft at its extreme anterior end also inter- 

 digitating with the posterolateral wing of the vomer. 

 Posteroventrally the prefrontal articulates through car- 

 tilage with the dorsal surface of the parasphenoid below. 



Parasphenoid. — An elongate shaft with a well- 

 developed keel along its ventral surface in the region of 

 the orbit. A deep, anteriorly directed cleft is present in 

 the posterior portion of the parasphenoid, giving rise to 

 the forked region which broadly overlies and inter- 

 digitates with the basioccipital and at the same time 

 covers over the medial groove on the basioccipital 

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

 the way back the parasphenoid possesses paired dorso- 

 lateral wings which interdigitate with the anteroventral 

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

 anterior edge of the myodome, while more posteriorly the 

 parasphenoid forms the floor of the myodome. Posterior 

 to its articulation with the prootics, the parasphenoid 

 overlies and interdigitates with the basioccipital. Just 

 below its region of articulation with the anteroventral 

 ends of the prootics, the lateral surface of the para- 

 sphenoid is slightly expanded into a knob, anterior to 

 which the keeled portion of the bone is slightly concave. 

 It is to this concavity that the upper elements of the 

 branchial arches are held by fibrous tissue, especially the 

 epibranchials and pharyngobranchials. The first 

 pharyngobranchial or suspensory element articulates 

 through fibrous tissue to the lateral surface of the para- 

 sphenoid in this region. In the region of the orbit the 

 parasphenoid is slightly arched dorsally and the keel is 2 

 to 3 times the depth of the more rounded shaftlike por- 

 tion above it. At its extreme anterior end the ventral sur- 

 face of the parasphenoid is somewhat concave to receive 

 and interdigitate with the posterior shaftlike portion of 

 the vomer, while dorsally at its anterior end the para- 

 sphenoid articulates through cartilage with the pre- 

 frontals £md ethmoid. 



Pterosphenoid. — Cartilage filled along its lateral 

 edges; articulates by interdigitation dorsally with the 

 frontals, posteriorly with the sphenotic, and ventrally 

 with the prootic. Along the middle of its medial edge the 

 pterosphenoid possesses a medial extension which meets 

 with that of its opposite member to interdigitate in the 

 midline. 



Ethmoid Region. 



Ethmoid. — The ethmoid remains largely car- 

 tilaginous, the ossification being mostly restricted to the 

 surface regions. However, the ossification is more ex- 

 tensive than in triacanthodids. The bone is widest along 

 the middle of its lateral surface where it interdigitates 

 with the long forward extensions of the prefrontals. 

 Posterodorsally the ethmoid interdigitates with the over- 

 lying anterior end of the frontal. Posterolaterally it ar- 

 ticulates through cartilage and some interdigitation with 

 the prefrontals, and ventrally through cartilage with the 

 dorsal surface of the parasphenoid and shaftlike portion 

 of the vomer. Anteromedially the ethmoid has a pro- 

 longation which interdigitates with the posteromedial 

 region of the dorsal surface of the vomer. The surface of 

 the medial region of the anterior one-fourth of the eth- 

 moid is somewhat thickened, this being the region over 

 which the premaxillary pedicel slides in the opening and 

 closing of the mouth. 



Vomer. — The vomer is expanded laterally at its 

 anterior end, while its flattened shaftlike posterior ex- 

 tension fits into and interdigitates with a concave area on 

 the ventral surface of the petrasphenoid. Posterolaterally 

 the vomer has a pair of extensions which extensively in- 

 terdigitate with the anterior ends of the extensions from 

 the prefrontals alongside the anterior end of the eth- 

 moid. The dorsal surface of the vomer helps support the 

 premaxillary pedicel, while laterally the expanded an- 

 terior region of the vomer articulates by tough fibrous tis- 

 sue with the medial surface of the middle region of the 

 palatine. Posteromedially the vomer interdigitates with 

 the anterior end of the parasphenoid, while the dorsal 

 surface of the posterior shaft of the vomer abuts against 

 the ethmoid cartilage above it. 



Mandibular Region. 



Hyomandibular. — A stout rectangular shaft whose 

 dorsal end firmly articulates by fibrous tissue with the 

 groove on the lateral surface of the underside of the skull 

 formed anterolaterally by the sphenotic, anteromedially 

 by the prootic, and posteriorly by the pterotic, the latter 

 bone also having a ventral flange which broadly overlies 

 the posterodorsal region of the lateral surface of the 

 hyomandibular. A well-developed crest for muscle at- 

 tachment is present about midway along the lateral sur- 

 face of the hyomandibular. Posteriorly the hyoman- 

 dibular articulates by fibrous tissue along most of its 

 length to the anterior edge of the upper region of the 

 preoperculum, just above the end of which latter bone 



