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Show 1905.] PRIMITIVE REPTILE PROCOLOPHON. 215 pelvis develops into the triradiating type such as is seen in Sphenodon, and in all the Diapsidan reptiles with the exception of the early Diaptosaurians the pelvis is a modification of this triradiating type. There probably were in Permian times large numbers of lizardlike reptiles which retained the roofed temporal region even after the shoulder-girdle and pelvis had become specialised, as it would be impossible to derive the Plesiosaurs and the Ichthyosaurs from two arched forms; and the Chelonians have evidently been specialised from a form which never had temporal arches at all, and yet had the Sphenodon type of shoulder-girdle and pelvis. The question then arises, are we to regard such reptiles as Cotylosaurians, or are we, in consideration of the fact that they are distinctly specialised along the Diapsidan line, to put them among the Diaptosaurians, as has been done by Osborn ? It is the same question as arises in connection with the classification of many groups of extinct forms; and I am inclined to agree with Osborn in placing in one group the whole phylum which has become specialised along one line, even though the early forms resemble the generalised members of the ancestral order more than they do the terminal forms. When we consider Procolophon, we find that while it bears considerable resemblance to Pariotichus, and even some resemblance to Pareiasaurus, it nevertheless seems distinctly specialised along the line which gave rise to Sphenodon. It still retains the roofed temporal region, the precoracoid, and the platedike pelvis, but it resembles Sphenodon in the arrangement of the bones of the temporal region, in the structure of the palate, in the structure and arrangement of the bones of the lower jaw, in having the teeth anchylosed to the bone, in the possession of intercentra, of which the anterior are paired as in the young Sphenodon, in having a well-developed plastron of abdominal riblets, and in the very close agreement of the structure of the carpus, tarsus, and phalanges. The bones of the temporal region have been variously identified by different authorities, and unless the squamosal is rightly determined, the other bones cannot be understood. The squamosal must be the bone that is the homologue of the mammalian squamosal, which, when we trace down among the Theriodonts and Anomodonts, we find to be the bone which supports the quadrate, and is itself supported by the parietal. When two bones are present in the temporal region, it is found to be always the inner which fulfils the condition-prosquamosal being, perhaps, the best name for the outer. In Procolophon the bone which seems to be undoubtedly squamosal is the one immediately above the quadrato-jugal, and this is the one which has been regarded as squamosal by Seeley and Osborn. This bone supports the quadrate as in Sphenodon, and is itself fixed to the parietal. The upper and outer bone, which is regarded by Dr. A. S. Woodward (11) as the squamosal, has no connection with the quadrate, |