374 Mr, T. Atthey on Pteroplax cornuta. 



to near the margin of the cranium. The wall of the cranium 

 is very thick around the foramen ; and from this to the occiput 

 there extends on each side a broad raised space with a depres- 

 sion outside of it ; the region of the posterior external angle is 

 thick and very strongly ossified, as are also the epiotic horns. 

 At the posterior end of the orbital curves, and corresponding 

 to a part of the postorbital bones at their under surface, is a 

 rough pitted space which looks like the articular surface for 

 another bone, which is lost, but which may have formed the 

 posterior border of the orbits. 



It is possible this skull may have been much decomposed 

 before its entombment. 



The dentition of Pteroplax has yet to be discovered ; and no 

 mandible has yet been obtained. 



Vertehrce. — These are not figured, being imperfect. On a 

 piece of black shale 5\ inches long by 3^ inches broad, in 

 my collection, are imbedded three vertebral centra and por- 

 tions of two vertebral processes. Two of the centra are much 

 less than the third, which measin-es in height -y^ inch, in 

 breadth lyV inch, in length ^ inch ] its sides are slightly exca- 

 vated, it is biconcave, and has a small notochordal foramen. 

 One of the processes, a transverse one, is j-^ inch long by the 

 same in breadth, and is much crushed. A zygapophysis is 

 present, but too much injured to admit of description. The 

 remaining process is of about the same size and in the same 

 condition. 



Ribs. — On the same piece of shale there are fragments of 

 three ribs ; the proximal ends of two are present, and, though 

 much flattened, show well the head and tubercle ; the distal 

 end of the third exists, but is much flattened. It is the under 

 surface, somewhat concave, which is exposed. In close rela- 

 tion with the head are three portions of ribs : one large frag- 

 ment, showing head, tubercle, and groove, lies on the left side 

 of the cranium ; another fragment with head and tubercle lies 

 obliquely under the cranium ; a short piece of the sternal 

 end of a rib rests in part on the occiput. 



These are shown in Plate XII. The first is 5^ inches long 

 by j^^ inch broad ; it is well and regularly arched ; the curve 

 of the bone is continued as far as the head, which ends in a 

 concave articular surface ; the tubercle, y^ inch posterior to the 

 head, ends likewise in a concavity for articulation. A groove 

 nins along the under surface from near tlie tubercle for two 

 thirds of the length of the rib ; beyond this the surface is 

 flattened. 



Tlie head, tubercle, and part of the groove are seen on the 

 rib Avliich lies under the cranium. Nothing is worthy of note 



