96 ELOPIFORM FISHES 



Description. The description of the neurocranium is based mainly upon the 

 acid prepared specimen. Several points were checked against a similarly prepared 

 specimen of 0. levis Woodward, B.M.N.H. P. 36240. The differences between these 

 two species are noted in the remarks on 0. levis. 



The head plus opercular apparatus is equal to 25 per cent of the standard length. 

 The maximum depth of the head occurs at the level of the quadrate/mandibular 

 articulation and is equal to 60 per cent of the head length. The diameter of the orbit 

 is slightly less than the preorbital distance and represents 20 per cent of the head 

 length. The bones of the cranium are thick and the dermal elements are coarsely 

 ornamented. 



Neurocranium. The neurocranium is long and shallow, with the cranial vault 

 occupying the posterior third of its length. The greatest width occurs at the 

 occiput and is equal to 50 per cent of the total neurocranial length. In lateral 

 profile the roof is comparatively straight. 



Anteriorly the roof of the cranium is formed by a large dermethmoid which, 

 unlike the other roofing bones, is unornamented and smooth. The anterior portion 

 of the dermethmoid is diamond-shaped, with the two antero-lateral margins in close 

 juxtaposition with the premaxillae. In some specimens there are two to four pores 

 aligned transversely across the anterior region of the dermethmoid. This line of 

 pores is interpreted as evidence of a bone-enclosed ethmoid commissure. Pos- 

 teriorly the dermethmoid extends backwards to the level of the lateral ethmoid and 

 is in syndesmotic union with the frontals, the suture line being ' W '-shaped. 



Each frontal is narrow anteriorly but expands above the posterior third of the 

 orbit. Much of the cranial vault is roofed by the frontal which is in contact with the 

 parietal posteriorly and the pterotic posteriorly and laterally. The frontal of either 

 side is united with its partner by a sinuous suture, the meanderings of which increase 

 posteriorly. The path taken by the supraorbital sensory canal within the bone is 

 marked externally by a strong ridge running the length of the orbital section, but 

 posteriorly the ornamentation obscures it. Ornamentation on the frontal (as with 

 the other roofing bones) takes the form of irregular tubercles arranged in sinuous, 

 sometimes anastomosing lines, radiating from the centre of ossification. The 

 ornamentation becomes less conspicuous anteriorly. 



The parietal, which joins its partner in the mid-line throughout its length, is 

 rectangular, with the shorter axis transverse. The posterior limit of the parietals 

 is such that the supraoccipital is excluded from the skull roof. Ornamentation is 

 similar to that on the posterior region of the frontal. 



The pterotic has three external faces ; dorsal (horizontal), lateral and posterior 

 (both vertical). The dorsal face, which represents the dermal component, shows 

 heavy ornamentation. The roof of both the dilatator fossa and post-temporal fossa 

 is formed by this face. A line of pores running along the lateral edge of the dorsal 

 face marks the path of the otic division of the cephalic sensory canal system. Pos- 

 teriorly the sensory canal leaves the bone by a single large pore. 



The lateral face of the pterotic forms the posterior two-thirds of the hyomandibular 

 facet, part of the inner wall of the dilatator fossa and the dorsal region of the sub- 

 temporal fossa. Medially this part of the pterotic forms the lateral wall and floor 



