Mr. T. Atthey on Pteroplax cornuta. 371 



The prolonged and narrow anterior end of our specimen 

 terminates abruptly at a somewhat oblique transversely frac- 

 tured surface. 



The frontal bones constitute nearly the whole of this narrow 

 part of the skull, which appears slightly deviated from the 

 straight line, as if it had received after death a blow on 

 the left side of the anterior end, which had started the poste- 

 rior end somewhat from its articulation with the parietal 

 bones. This part is rather broader at its distal end than else- 

 where. 



The frontals are united to each other on the median line ; 

 behind and at each side of that line they form a retreating 

 angle, into which are received the united ends of the parietals ; 

 external to this they articulate with the postfrontals, the pos- 

 terior extremity of each frontal being received into an angle 

 formed by the parietals and postfrontals. 



I'he parietals occupy the chief part of the middle region of 

 the cranium, and are of a triangular form, the base being pos- 

 terior. In the middle of the median suture is the oval parietal 

 foramen. It is large, being y^o inch in length and rather 

 more than yij inch in breadth ; it is surrounded by an elon- 

 gated oval space, which is somewhat elevated, the bone here 

 being thick and strong ; and outside of this is a depression of 

 the same form, gradually rising outwards to the ordinary level 

 of the surface of the skull. At the anterior part of the parie- 

 tals the median suture has an irregular course to the left, no 

 doubt a mere individual irregularity. On their outer borders 

 these bones articulate, from front to back, successively with 

 the postfrontals, the postorbitals, the squamous, the epiotic, 

 and the so-called supraoccipitals which lie behind them. 



The postfrontals are narrow, curved, and elongated, pointed 

 in front, where they abut upon the frontals, and broader 

 behind, where they meet the postorbitals ; their internal bor- 

 ders, irregularly convex, articulate with the frontals and parie- 

 tals ; their external borders form the greater part of the con- 

 cave and smooth inner margin of the orbits. 



The prefrontals^ which may have formed the anterior part 

 of the inner margin of the orbits, and the lachrymals are 

 absent. 



The postorbitals^ articulating in front with the postfrontals, 

 internally with the parietals, and posteriorly with the squa- 

 mous, are short thick bones which form the posterior part of 

 the inner orbital margin, and present an external projection 

 which marks the termination of that margin and to which, as 

 well as to the similar process on the postfrontal, ligaments 

 and other structures forming the outer wall of the orbit were 



2G* 



