1905.] PRIMITIVE REPTILE PROCOLOPHON. 215 



pelvis develops into the triracliating type such as is seen in 

 Sphenodoii', and in all the Diapsidan reptiles with the exception 

 of the early Diaptosanrians the pelvis is a modification of this tri- 

 radiating type. 



There probably were in Permian times large numbers of lizard- 

 like 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 Sphenoclon 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 Diaptosanrians, 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 f ornis 

 resemble the generalised members of the ancestral order more than 

 they do the terminal forms. 



When we consider Procolojihon, we find that while it bears 

 considerable resemblance to Pariotichids, and even some resem- 

 blance to Pareiasaurus, it nevertheless seems distinctly specialised 

 along the line which gave rise to Spheyiodon. It still retains the 

 roofed temporal region, the j)recoracoid, and the plate- like pelvis, 

 but it resembles Splienodon 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 anteiior 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 cai-pus, tarsus, and 

 phalanges. 



The bones of the temporal region have been variously identified 

 by difi'erent authoi-ities, and unless the squamosal is rightly de- 

 termined, the other bones cannot be understood. The squamosal 

 must be the bone that is the homologue of the mammalian 

 squamosal, which, when we ti'ace down among the Theriodonts 

 and Anomodonts, we find to be the bone which supj)orts 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 Sj^henodon, and is itself fixed to the parietal. 

 The upper and outer bone, which is regarded by Dr. A. S. Wood- 

 ward (11) as the squamosal, has no connection with the quadrate, 



