122 University of Kansas Geological Survey. 



and the basioccipital, is largely excavated for the semicircular 

 canals. The sutural surface for the petrosal extends narrowly 

 along the upper side of the squamosal, and more narrowly be- 

 low it, thus inclosing in a long, conical cavity the inner process 

 of the squamosal. 



Are not these relations of the squamosal, wedged in between 

 the petrosal and the exo-paroccipital, anomalous among rep- 



tilia? 



Petrosal ( pvootic). 



Platecarpus. The petrosal unites by a short, flattened, sutu- 

 ral surface with the exterior part of the basioccipital and by a 

 longer, similar one, with the basisphenoid. The thickened an- 

 terior margin is emarginate near its middle for the trigeminal 

 nerve, a protuberance superiorly sometimes partly inclosing 

 the notch into a foramen. Just back of this emargination, on 

 the inner side, is the opening for a small foramen. On the 

 outer side, near the posterior margin, there is an elongated slit 

 covered by a thin scale of bone, having a small foramen at its 

 bottom The surface for union with the parietal is shorter than 

 that for the supraoccipital, which it meets in nearly a right angle. 

 The groove for the stapes is dilated somewhat beyond the foramen 

 leading into the semicircular canals. These openings are chiefly 

 excavated from the petrosals, with a smaller excavation upward 

 into the supraoccipital and another into the exoccipital. The 

 sutural surface for the parietal is flatly truncated, and was evi- 

 dently covered with cartilage for union with the thin, decurved 

 margin of the descending wing of the parietal. The anterior 

 part of this surface, continuous with the anterior border as far 

 as the trigeminal notch, is markedly roughened for ligamentous 

 attachments. The petrosal in general is triradiate in form, of 

 which the stouter branch is for articulation with the basioccip- 

 ital and basisphenoid, the longest for union with the exoccip- 

 ital, covering the process of the squamosal, as described in that 

 bone. The smallest process is for the union with the supra- 

 occipital and parietal. The external branch is flattened dis- 

 tally, lying upon and in front of the exoccipital and reaching 

 nearly to its distal extremity. Rarely is the bone found sepa- 



