182 MK. T. ATTHKY ON rXEliOPLAX CORNUTA. 



less cartilaginous or loosely attached ; the presphenoid and sphe- 

 noid median ridge and the basioccipital, which are seen in the 

 skulls of Loxomma and Anthracosaunis, are also gone. The quad- 

 rates at the base of the epiotic horns are the only bones of the 

 base of the skull that remain. The horns are a good deal da- 

 maged. The ai-ticular condyle for the mandible is not to be seen. 

 The under surface of the frontal bones is grooved on each side of 

 a median ridge, along their whole length, as if they had formed 

 the roof of a double nasal cavity extending from the snout to the 

 throat. 



The parietal foramen is large, open, and funnel-shaped, widen- 

 ing out gi-eatly as it passes through the thickness of the cranium ; 

 it is here six-sixteenths of an inch long and four-tenths of an 

 inch broad, whilst on the upper surface of the skull it is only a 

 quarter of an inch long by two-tenths of an inch broad. Its 

 margin is suiTOunded, except in front, by a sharp ridge of bone, 

 from which pass off laterally smaller ridges, which, di^-iding, 

 enclose small smooth depressions that extend 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 depression outside of it ; the re- 

 gion of the posterior cxteraal angle is thick and very strongly 

 ossified, as arc 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 

 whicli may have formed the posterior border of tlie orbits. 



It is possible this skull may have been much decomposed be- 

 fore its entombment. 



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

 mandible has yet been obtained. 



Vertehrcc.— These are not figured, being imperfect. On a piece 

 of black Hhale, five inches and a half long by three inclies and a 

 half broad, in my collection, are imbedded the vertebral centra 

 and portions of two vertebral processes. Two of the centra arc 

 much less than the third, which measures in height nine-tenths 



