1 \ OLUTIONARY SUMMARY 25 



In the chameleon, the crest is Formed by the middle parietal bar reinforced from below at its 

 anterior end by the supraoccipital. Here, the parietal broadens out to meet the postfrontal or post- 

 orbital, which in turn articulates with the jugal below, and the squamosa] below and behind. The 

 latter bone, passing backward, arches inward until it meets the hinder end of the parietal bar, thus 

 enclosing an opening interpreted as the supratemporal fossa, but equivalent in position to the parietal 

 fenestra of the Ceratopsia. In the latter, the supratemporal fossa is separated from the fenestra by 

 a transverse portion o( the parietal, but the two apertures may once have been confluent before the 

 formation of the transverse portion, which in all large fenestrated forms still lies below the level 

 of the remainder of the crest and forms the area for the origin of the temporal muscles. 



In the Protoceratopsidae, a sagittal crest indicates that the origin of the temporal muscles over- 

 lay the greater part if not all of the dorsal surface of the parietal portion of the frill. In the Cera- 

 topsidae, there are definite but limited areas in the wake of the supratemporal fossae which received 

 the origin of this muscle. Thus, the frill had a dual function, mechanical and protective, for it formed 

 the origin of the temporal muscles of the jaws on the upper aspect, while below it served for the 

 insertion of the several neck muscles which wielded the head. It also formed a protective buckler to 

 guard the otherwise unprotected portion of the spinal cord from the point of its emergence from the 

 foramen magnum until it disappeared beneath the neural arch of the axis, a distance of several inches. 

 Other vital portions of the neck, such as arteries and veins, were afforded protection as well. 

 Initially, the formation of the frill was analogous to that of the chameleon; but whether the same 

 elements enter into its formation in each instance is a matter of debate. 



The dorsal surface of the cranium, while unmodified in the Protoceratopsidae, even in the adult, 

 is altered in the Ceratopsidae through the formation of a false roofing analogous to that found in 

 the Chelonia, and inclosing spacious sinuses between the roofing bones and the brain case proper. 

 This is doubtless correlated with the evolution of the supraorbital horns, yet its development 

 seems to have anticipated that of the horns, for it is well established, although not perfected, in 

 genera in which these horns may be in an incipient stage, such as in some species of Chasmosaums 

 and in Styracosaurus. Another peculiar feature which is the outgrowth of this false roofing is the 

 postfrontal fontanelle which lies in the midline of the dorsal aspect of the cranium, just above the 

 level of the supratemporal fossae. This forms an opening into the sinuses from without and was 

 supposed by Marsh to be the pineal foramen. This it clearly is not, for it does not communicate 

 with the endocranial space, but what function it actually served we do not know. When I 1 gave 

 the name of postfrontal fontanelle to it, I supposed that its presence was temporary and that it always 

 closed over completely with age. But I find this is not true, for in but a single instance known to 

 me, that of the type of Trkeratofs prorsus, is the closure complete. Moreover, there are always, so 

 far as I know, two canals which radiate outward and backward from the fontanelle and of which 

 vestiges are discernible even in the prorsus skull. This would seem to imply some sort of function, 

 but not that of the emission of bloodvessels on the surface of the crest, for the vascular impressions 

 emerge from the supratemporal fossae, but are not apparent until the margin of the smooth area for 

 the origin of the temporal muscle is reached, where they are few and deep, ramifying into a countless 

 number as they disperse over the crest. 



Hay in speaking of the postfrontal fontanelle says, "Probably in all cases this postfrontal 

 foramen divides below into two, one for each side," as Hatcher describes in Torosaurus latus. "It 

 seems perfectly clear that the foramen in question represents the supratemporal fossae of the alliga- 

 tor and of various other reptiles. The bone rising up and dividing the fossa into two, either at the 

 surface or deeper down, will almost certainly be found to be the true parietal. Through the enor- 

 mous thickening of the postfrontals four parietals] the parietal has been crowded from the upper sur- 

 face of the skull of Trkeratofs and the two supratemporal fossae have been pushed into one at the 

 midline." 2 But we recognize the supratemporal fossae as present and functional. Perhaps the canal- 

 like depressions radiating from the postfrontal fontanelle represent old lines of communication 



1 Hatcher, Marsh, Lull, 1907, p. 24 (footnote). 



2 Hay, O. P., 1909, pp. 97-98. 



