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Show 1905.] PRIMITIVE REPTILE PROCOLOPHON. 221 below the postsquamosal above, and between the squamosal and quadrate bones behind it and the postorbital in front, so that the space between the bones, which might be occupied by the supra- ' temporal, has only a linear extension on the external surface, above the malar. The internal suture which separates a supratemporal from the squamosal is not clear in specimens of other species. The Postfrontal.-One of the most characteristic features of Procolophon is the small size of the postfrontal bone, which is a narrow strip above the orbit external to the parietal and frontal, contrasting with the relatively large size of the prefrontal bone. There is an appearance of the prefrontal and postfrontal both underlapping the frontal bones in P. trigoniceps, but the preservation is dissimilar on the right and left side of the head, and the evidence is not conclusive that the postfrontal is larger than it appears to be. This character is in marked contrast to the condition in Dicynodontia and Theriodontia, in which the postfrontal not only contributes to the bar which divides the orbit from the temporal vacuity, but is prolonged backward on the temporal vacuity along the bevelled margin of the parietal bone. I conclude, from detailed comparison of these structures and from measurements, that the type species are founded upon characters which clearly distinguish them. Other evidence shows unexpected variation in the skulls o f Procolophon. In 1878 I described additional material also from Donnybrook, and discussed the affinities of the genus with Hatteria and Ano-modont reptiles. Three species appeared to be indicated by as many specimens, and were described under the names P. griersoni P. laticeps, and P. cuneiceps, and figured in pi. xxxii. Quarterly Journal Geol. Soc. vol. xxxiv. The matrix was afterwards further removed from these fossils, chiefly in the endeavour to elucidate the back of the skull and the quadrate region. The published figures, which are somewhat rough, are chiefly directed to show external variations of form, and the divided nares. Beyond correcting the identification of the postfrontal bone in the way already indicated in the evidence figured in 1889 (Phil. Trans. Roy. Soc. B. pi. 19), and omitting the quadrato-jugal bone, I have nothing to modify in those descriptions ; but better specimens would be required to prove that the characters in which they differ are constant. The Occipital Region.-Although all these types were developed to display the occipital region, it was only found in Procolophon laticeps. The transverse, slightly concave occipital border of the roof of the skull, formed by the parietal bones and postsquamosal bones at the outer angles (text-fig. 31), extends backward as a led^e beyond the nearly vertical occipital aspect of the skull, which it slightly overhangs. The ledge is inclined downward, and terminates in a sharp edge, which at the outer angles curves down with the postsquamosal to form an arch above the auditory notch behind the squamosal bone (text-fig. 32). |