FOSSIL AND RECENT 113 



elements, whereas in 0. levis there is an interspace. The interorbital septum is not 

 completely ossified in 0. levis in contrast to the well-ossified septum in 0. lewesiensis. 

 Posteriorly the myodome is open in 0. levis whereas in the type-species it is closed. 



Apart from the differences noted above, the neurocranium is very like that of the 

 type-species. Woodward (1907 : 118) described much of the cranial anatomy 

 other than the neurocranium, and the following notes, based upon an examination 

 of B.M.N.H. P. 36240, are intended to supplement that work. 



Hyopalatine bones. The hyomandibular tapers from a broad undivided head to a 

 vertically directed shaft, unlike the forwardly directed shaft of the type-species. 

 The opercular process is large, rounded and bears a stout horizontal ridge upon its 

 medial surface. This ridge provided a point of insertion for the levator opercularis 

 muscle. Laterally the shaft of the hyomandibular bears a stout ridge behind which 

 there is a well-marked groove. Two foramina open into the groove from a single 

 intraosseous canal, the upper representing the point of exit of the hyoidean nerve, 

 the lower the mandibular nerve. The nerves continued within the hyomandibular 

 groove. As in the type-species, the anterior margin of the hyomandibular is pro- 

 duced into a thin wing which is partly overlain by the metapterygoid. 



The remainder of the hyopalatine series is very similar to that of 0. lewesiensis, 

 the only notable difference being that the quadrate is deeper than long in 0. levis, 

 which is the converse of the type-species. 



The hyoid arch and branchiostegal rays. The interhyal was not identified in any 

 specimen, although its presence is indicated by a notch upon the dorsal surface of 

 the posterior ceratohyal. The posterior ceratohyal is approximately triangular, 

 smooth upon the medial surface, but marked laterally by two depressions separated 

 by an antero-ventrally inclined ridge. The anterior ceratohyal is elongate and 

 waisted in the anterior third. It is deepest posteriorly where it contacted the 

 posterior ceratohyal through a narrow band of cartilage. A prominent groove 

 close to the dorsal margin of the lateral surface of the anterior ceratohyal marks 

 the path of the afferent hyoidean artery. Ventrally the anterior ceratohyal shows a 

 groove flanked medially by a notched ledge. Each of these notches marks the point 

 of insertion of a branchiostegal ray suggesting that not less than five branchiostegals 

 were attached along this margin. 



There were two hypohyals, although only the upper is known. This is cuboid in 

 shape and was separated from both the ceratohyal and lower hypohyal by cartilage. 

 The groove for the afferent hyoidean artery runs along the dorsal surface of the 

 upper hypohyal. In the type species the artery runs through the upper hypohyal. 



The median urohyal is ' T '-shaped in cross-section. Anteriorly the horizontal 

 lamina of bone is constricted before becoming expanded into two antero-laterally 

 directed prongs. The median vertical flange of bone is thin, but its posterior extent 

 is not known. 



Not less than 15 branchiostegal rays are present. The anterior branchiostegals 

 are narrow but the posterior rays are broad and marked by a stout ridge which runs 

 down the centre of their length. 



Gill arches. Very little is known of the branchial arches in this or the type-species. 

 The basihyal and basibranchials were covered by two large tooth plates, the basihyal 



8 



