expanded along the anterior third of its ventral edge to 

 form a slightly concave horizontal plate over the roof of 

 the oral cavity. The parasphenoid articulates by inter- 

 digitation posterolaterally with the prootics, while at its 

 extreme posterior end it broadly overlies and interdigi- 

 tates with the basioccipital. At its blunt anterior end the 

 parasphenoid is deeply and firmly interdigitated with 

 both the ethmoid and vomer, and in large specimens 

 (over approximately 200 mm) these articulations become 

 truly fused. Anterodorsally the parasphenoid interdigi- 

 tates with the ethmoid, while dorsally in about the mid- 

 dle of its length the parasphenoid becomes slightly ex- 

 panded laterally and articulates by fibrous tissue and, in 

 large specimens, by interdigitation with the bases of the 

 prefrontals. In the anterior half of the orbital cavity the 

 parasphenoid is present as a thin flat plate, the extreme 

 posterodorsal edge of which makes contact, in large 

 specimens at least, with the extreme anterior end of the 

 pterosphenoid. In large specimens the articulation of the 

 parasphenoid and pterosphenoid is by interdigitation, 

 whereas in smaller specimens the articulation is through 

 the interorbital sheet of fibrous tissue. In about the mid- 

 dle of its length, where it is laterally expanded, the para- 

 sphenoid makes contact by interdigitation with the 

 anterior prolongation of the prootics. 



Pterosphenoid. — Cartilage filled along all of its 

 edges of articulation with the other cranial bones, except 

 anteriorly; articulates by interdigitation posterodorsally 

 with the somewhat overlying frontal, posteriorly with the 

 sphenotic and posteroventrally with the prootic. The 

 anterior edge of the pterosphenoid is prolonged anteriorly 

 into a flattened process which makes contact with the 

 posterior edge of the platelike orbital expansion of the 

 parasphenoid, as explained above. 



Ethmoid Region. 



Ethmoid. — Elongate; a rounded shaft posteriorly, 

 but becoming laterally expanded anteriorly; cartilage 

 filled at its posterior edge, where it is continuous with the 

 ethmoid cartilage; articulates by interdigitation ventral- 

 ly along the posterior two-thirds of its length with the 

 parasphenoid, while ventrally in the anterior one-third of 

 its length it firmly articulates by extensive interdigi- 

 tation with the vomer. In large specimens the articu- 

 lation between the ethmoid and vomer becomes fused. At 

 its anterior edge the ethmoid helps support, through 

 fibrous tissue, the upper jaw. 



Mandibular Region. 



Hyomandibular. — Much expanded posteriorly, 

 tapering gradually to a blunt end anteriorly; cartilage 

 filled at its posterior and anterior edges; articulates by 

 fibrous tissue posteriorly with the slight concavity on the 

 anterior edge of the laterally expanded posterior portion 

 of the prootic and, for a short distance, with the con- 

 tinuation of that concavity on the anteromedial edge of 

 the pterotic, as well as along the medial surface of the 

 ventral shaft of the pterotic. The dorsomedial edge of the 

 hyomandibular is also supported through fibrous tissue 

 by the lateral surface of the ventral flange of the prootic 

 shelf. The hyomandibular possesses a slight concavity on 

 its ventral edge in which the rounded dorsal end of the 

 operculum is held by fibrous tissue. Just anterior to this 

 articular area the hyomandibular bears an elongate 

 groove on its lateral surface into which the slender pos- 

 terodorsal end of the preoperculum fits and is tightly 

 held by fibrous tissue. Anteriorly the hyomandibular is 

 attached to the fibrous tissue sheet between the 

 metapterygoid, symplectic, and preoperculum. 



Quadrate. — Wide posteriorly, tapering anteriorly 

 to a knob for articulation with the articular in the lower 

 jaw, while from its posteroventral edge it possesses a 

 short posteriorly directed process; cartilage filled at its 

 posterior edge; articulates by interdigitation dorsally 

 with the ectopterygoid, posterodorsally with the 

 mesopterygoid and posteroventrally with the symplectic. 

 Along the posterior two-thirds of its ventral edge the 

 quadrate articulates by fibrous tissue with the preoper- 

 culum. 



Metapterygoid. — Elongate; wide anteriorly but 

 becoming narrower posteriorly; cartilage filled at its 

 anterior edge; articulates by interdigitation anterodor- 

 sally with the mesopterygoid and anteroventrally with 

 the symplectic, which it somewhat overlies. Posteriorly 

 the metapterygoid articulates by fibrous tissue with the 

 hyomandibular and interhyal. 



Symplectic. — Slender and elongate; no cartilage 

 filled edges evident; articulates by interdigitation an- 

 teriorly with the quadrate, posteriorly with the 

 metapterygoid and by fibrous tissue posteroventrally 

 with the preoperculum and hyomandibular. 



Palato-Pterygoid Region. 



Vomer. — Short, slightly expanded laterally in 

 about the middle of its length; articulates by interdigi- 

 tation dorsally with the ethmoid and posteriorly with the 

 parasphenoid, but in large specimens both of these ar- 

 ticular surfaces become fused. Anterolaterally, just in 

 front of its laterally expanded region, the vomer supports 

 through fibrous tissue the palatines, while at its extreme 

 anterior end it supports the upper jaw. A slight concavity 

 in the lateral surface of the vomer marks the place of ar- 

 ticulation between the vomer and palatine. 



Palatine. — A small wedge of bone; articulates by 

 interdigitation ventrally with the ectopterygoid and, to a 

 lesser extent, with the mesopterygoid. Even though the 

 palatine has often been said to be absent in ostraciids, it 

 is distinct from the ectopterygoid even in large specimens 

 and the suture between the two can be seen upon close 

 examination. Dorsomedially the palatine articulates by 

 fibrous tissue from a slightly rounded prominence on its 

 upper medial surface with the slight concavity on the 

 lateral surface of the vomer. 



