18 POSTERIOlt CRANIAL ARCHES 



satus Cope,* I find the bone in front of the arch to form two Hmbs of a nearly right- 

 angled triangle, one of which is supraorbital, and the other postorbital. JSTo suture 

 divides it. It may represent the fused postfrontal and postorbital elements which we 

 have in some Lacertilia. There is, however, a small free bone horizontally placed at 

 the internal side at the posterior extremity of the supraorbital limb, which may be a 

 postfrontal bone. In this case the anterior connection of the supramastoid bone will 

 then be with the postorbital. This must however be clearly proven before it can be 

 accepted, since it is the postfrontal bone f which articulates with the supramastoid 

 posteriorly. If we suppose the long perpendicular postorbital process of the bone in 

 question to represent the postorbital bone of Ichthysaurus, the question is simplified, 

 but it is not certain that such is the case. 



The figures given by Marsh of the ^^ Geratosaurus " nasicornis represent a struc- 

 ture similar to that of Laelaps, and similarly indecisive. The figures of Hypsirliophus 

 stenojJsX (Stegosaurus Marsh) exhibit distinct postfrontal and postorbital bones. 

 They show the postorbital produced upwards and backwards to form the horizontal 

 bar with a posterior element. Between this element and the parietal is represented 

 on one side of the figure another element, but this entire region is left undescribed in 

 the text. The appearance given by one side of the figure (3) is that the supramas- 

 toid and supratemporal are both present, and that the latter is the posterior element 

 in the bar. In that case the structure is that of the Theriodonta and Lacertilia, and 

 not that of the Ichthyosauria. 



The situation in the Crocodilia appears to be the same as in the Dinosauria. 

 ^Nothing satisfactory can be learned from the recent members of the order ; and even 

 in the skull of an Alligator mississippiensis one inch long, the postfrontal and post- 

 orbital bones are not distinct from each other; The Jurassic forms of the Teleosau- 

 ridse show the same character, and give the appearance of a postorbito-supratemporal 

 arch.§ In the Triassic Belodon the structure seems to be essentially similar. The 

 appearance in the Pterosatjria, as figured b}^ authors, is the same as in the 

 Dinosauria, but I cannot pronounce decisively in the lack of specimens. It is not 

 unlikely that all the members of the Archosaurian series resemble each other in this 

 respect, and I suspect that it is to be explained by reference to the Theriodonta. 

 Here the postfrontal and postorbital are distinct, as already pointed out, but the for- 

 mer is small and is crowded by the adjacent elements. Its fusion with the post- 



* Which I owe to the Geological Survey of Cauada. 



f Including the supposed squamosal of my description of Diclonius {I.e.). 



t Amer. Jour. Sci. Arts., 1887, PI. VI. 



^ See Eudes Deslongchamps Notes Paleoutologiqucs, 1803-9. 



