370 Mr. T. Attliey on Pteroplax cornuta. 



Upper surface of the smaller cranium (PL XII. of natural 

 size and as seen after careful removal of the matrix). — A rib 

 is seen lying along upon the left side of the cranium ; and near 

 its distal end a fragment, which may have belonged to it, 

 partially overlaps the left occipital region ; a third piece, the 

 vertebral end of a rib, lies under the fore part of the skull. 



It is worthy of remark that the premaxillary, nasal, pre- 

 frontal, and maxillary bones, which naturally form the ante- 

 rior end of the cranium, are absent from all the three skulls of 

 Fteroplax as yet known. Such a deficiency has never been 

 met with in skulls of Loxomma or Anthracosaurus. The 

 skull of Pteroplax^ as we find it, consists of frontals, parietals, 

 so-called supraoccipitals, postfrontals, postorbitals, squamous, 

 occipitals, epiotics, and quadrate bones, all firmly united by 

 suture. 



Dimensions. — The skull thus composed measures in length, 

 from the median line in front to the posterior ends of the 

 epiotic horns, 6^ inches, and from the same point to the pos- 

 terior margin of the occiput on the median line 5 inches. The 

 distance from this last point to the point of the right epiotic 

 horn is very nearly 2 inches, and that between the points of 

 those horns 2^jj inches. Breadth of the skull at the occipital 

 margin 2k inches, at the broadest part 2^% inches, between the 

 posterior ends of the orbital curves 2| inches, between the 

 anterior ends of the same 1 inch, over the anterior broken 

 ends of the frontals rather more than 1 inch. 



The lyitted sculpturing is irregularly disposed over the sur- 

 face, the pits varying much in form, size, and depth, being on 

 the whole smaller and less deep than those of the crania of 

 Loxomma and Anthracosaurus ; on the parietal bones they 

 are prolonged into grooves radiating from the centres of ossi- 

 fication on each side of the parietal foramen. The pits are 

 most strongly marked on the postfrontals and postorbitals, 

 and are largest and groove-like on the frontals. On these 

 bones there is besides a channel or groove at each margin, 

 studded with pits, probably a mucus-groove. The pits on the 

 frontals are all inclined forwards, and look as if they served to 

 lodge blood-vessels. None of the small punctures which exist 

 at the bottom of the pits in Loxomma are visible in the pits of 

 Pteroplax, any more than in those of Anthracosaurus. 



The orbits appear to have been large for the size of the 

 skull ; their internal, concave, osseous margins, formed of 

 the postfrontal and postorbital bones, alone remain to testify 

 to this. They are placed where the skull rapidly tapers to 

 the elongated frontal region. The rest of their outline was 

 probably partly membranous or ligamentous and cartilaginous. 



