278 Dr. R. H. Traquair on the Structure 



the outer tooth-bearing ridge, is slightly folded outwards, so 

 as to form a ledge along which the maxilla was no doubt 

 articulated ; about the posterior fourth of the margin this 

 ledge is suddenly cut off obliquely, probably to allow the 

 passage of the masticatory muscles to the lower jaw. 



The bone just described thus forms a powerful palato- 

 pterygoid plate, extending from the hyomandibular and the 

 articulation of the lower jaw to the front of the head, and, as 

 shown by radiating lines seen on impressions of its outer 

 aspect, was ossified from a centre placed below and behind, 

 close to the place of coalescence of the dentigerous ridges. 

 It is not, however, certain that it was the only osseous element 

 in the palatoquadrate arch, as I have in one head seen what 

 seem to be traces of another ossification near its posterior 

 superior angle. 



I have not seen the original specimen from which Dr. Young 

 took the figure which he gives as representing the edge of the 

 mandible as seen from above*. But on comparing that figure 

 (which has certainly nothing to do with the mandible) with 

 the bone we have just considered, there seems to me to be 

 little room for doubt that he has mistaken the tooth-bearing 

 edge of a fragmentary specimen of the palatopterygoid for 

 the dental margin of the mandible, and has placed its posterior 

 extremity forwards as well as its lower aspect upwards. 



Hyomandibular , Opercular , and Branchiostegal apparatus. — 

 In fig. 3 the form and connexions of the hyomandibular (hm) 

 are clearly displayed. It is an elongated bone passing down- 

 wards, and with a very slight inclination forwards, from the 

 side of the cranium below the squamosal towards the articu- 

 lation of the lower jaw, which, however, it does not reach, but 

 terminates a little above and behind it. It shows a marked 

 constriction a little above the middle ; the part above this is 

 laterally flattened and somewhat expanded ; below the con- 

 striction the bone is slender and cylindrical, and increases a 

 little in diameter as it proceeds downwards. There is no 

 symplectic ; and the upper element of the hyoid would be, as 

 in Potypterus, attached to the cartilaginous lower extremity of 

 the hyomandibular. 



Supported by the hyomandibular are the operculum (op) 

 and suboperculum (s.op). Both bones are somewhat quadrate 

 in shape and higher than long ; but the height of the sub- 

 operculum exceeds that of the operculum by about one fourth. 

 The posterior-superior angle of the operculum and the posterior- 

 inferior one of the suboperculum are rounded off, while their 

 * Op. cit. pi. xx. fig. 4. 



