digitation of the anterolateral edge of the prootic with the 

 anterodorsal edge of the hyomandibular. 



Orbital Region. 



Frontal. — Extremely large and laterally ex- 

 panded; articulates by interdigitation posteromedially 

 with the supraoccipital, which it slightly overlies, and 

 posterolaterally with the sphenotic. Medially along its 

 ventral surface in the region of the orbital cavity the fron- 

 tal interdigitates with the pterosphenoid. At its anterior 

 end the frontal is extensively interdigitated medially 

 with the palatine and laterally with the prefrontal, the 

 latter bone being broadly overlain by the frontal. Along 

 the posterior half of its medial edge the frontal interdigi- 

 tates with its opposite member, but more anteriorly the 

 medial edges of the two frontals articulate with one 

 another only by fibrous tissue. Between the regions where 

 the medial edge of the frontal is in close contact with its 

 opposite member and where it interdigitates with the 

 palatine, the frontal overlies the ethmoid cartilage. 



Prefrontal. — A thin plate of extremely variable 

 size, but always broadly overlain by the frontal and 

 hence apparently very small as seen dorsally. The pre- 

 frontal articulates by extensive interdigitation medially 

 with the palatine and posteriorly and laterally with the 

 frontal. 



Parasphenoid. — More or less in the form of a cross, 

 with the longer shaft becoming increasingly deeper and 

 wider toward its anterior end. The anterior end of the 

 parasphenoid is so deeply concave that the cavity ex- 

 tends back posteriorly almost to the level of the shorter 

 transverse arms of the parasphenoid. The parasphenoid 

 articulates by interdigitation posterolaterally along the 

 dorsal surface of its transverse arms with the prootics, 

 while at its extreme posterior end the parasphenoid over- 

 lies and interdigitates with the basioccipital. Between its 

 regions of interdigitation with the basioccipital and with 

 the prootic, the parasphenoid has its lateral edges in con- 

 tact with the cartilage that is present along the medial 

 edges of the prootics. Anterolaterally the parasphenoid 

 interdigitates with the medial edge of the palatine. The 

 highly concave anterior end of the parasphenoid has its 

 dorsal surface held to the ventral surface of the ethmoid 

 cartilage. The posterior end of the platelike ossification 

 that occurs between the ethmoid cartilage and the dor- 

 sal surface of the parasphenoid, to be described below as 

 the ethmoid-vomer, is slightly overlain by and interdigi- 

 tated with the anterior concave end of the parasphenoid. 

 A very shallow ventral flange or keel is present along the 

 medial portion of the ventral surface of the para- 

 sphenoid in the region under the orbital cavity. 



Ethmoid Region. 



Ethmoid-vomer. — All that remains of the ossifica- 

 tions of the ethmoid region is a plate of bone which is 

 very thin throughout its length, except at its anterior 

 edge where it becomes substantially thicker. The plate of 

 bone lies between the ventral surface of the ethmoid car- 

 tilage and the anterodorsal portion of the parasphenoid. 

 The posterior end of the plate fits under and interdigi- 

 tates with the inner surface of the dorsal half of the deep- 

 ly concave anterior end of the parasphenoid, while the 

 anterior edge of the plate lies anterior to the ethmoid car- 

 tilage. There is no evidence in any of the specimens ex- 

 amined that any of the edges of the plate are cartilage 

 filled, but even if the plate were entirely an endochon- 

 dral ossification the bone is so thin that one would not 

 expect to find cartilage filled edges. Only by histological 

 examination of the developing bone would it be possible 

 to state whether the bone is of dermal or endochondral 

 origin or of a combination of the two. From the position 

 of the anterior portion of the plate (between the frontals 

 and in at least close apposition with the ethmoid car- 

 tilage) one would suspect that the bone is the ethmoid. 

 However, the posterior end of the bone fits into the con- 

 cavity of the parasphenoid, a characteristic of the vomer 

 in most plectognaths. Until histological examination of 

 the development of the bone sheds light on its nature, I 

 presume that it probably represents the fused rudiments 

 of both the ethmoid and vomer. 



Mandibular Region. 



Hyomandibular. — Expanded dorsally, tapering to a 

 stout shaft anteroventrally; cartilage filled at its antero- 

 ventral edge and along the middle of its dorsal edge; ar- 

 ticulates dorsally by fibrous tissue with the concavity 

 along the ventrolateral edges of the prootic and sphenotic 

 and along the anterior half of the anteroventral edge of 

 the pterotic. The support of the hyomandibular is fur- 

 ther strengthened by the extensive interdigitation of its 

 anterodorsal end with the anterolateral edge of the 

 prootic and of its posterodorsal end with the ventro- 

 lateral edge of the pterotic. Anteriorly the hyoman- 

 dibular articulates by fibrous tissue with the posterior 

 ends of the metapterygoid and symplectic. In large 

 specimens the articulation between the hyomandibular 

 and the metapterygoid becomes slightly interdigitated. 

 Along the ventral half of its posterior edge the hyoman- 

 dibular articulates by fibrous tissue with the preoper- 

 culum. Immediately behind its uppermost point of con- 

 tact with the preoperculum, the hyomandibular 

 possesses an upraised region whose posterior surface is 

 hollowed out to receive and hold by fibrous tissue the an- 

 terior rounded articular facet of the operculum. 



Pterosphenoid. — Relatively elongate dorsoven- 

 trally; much wider dorsally than ventrally; cartilage 

 filled along all of its edges, except medially; articulates 

 by interdigitation dorsally with the frontal and ventrally 

 with the prootic. 



Quadrate. — Wide posteriorly, tapering to a knob 

 anteriorly for articulation with the articular in the lower 

 jaw; only a very short posterior process present from its 

 ventral edge between the symplectic and preoperculum; 

 cartilage filled at its posterior edge; articulates by inter- 



