OCR Text |
Show 220 PROF. H. G. SEELEY ON THE [Mar. 21. markings already referred to, which appear to indicate a posterior division of the parietal bones, really indicate bones, they would represent the pair of ossifications termed supraoccipital in Labyrinthodonts, over which the parietal bones may extend. Procolophon may thus far be crypto-Labyrinthodont in the structure of this part of the head. In Pareiasaurus there appears to be a narrow bone behind the parietal bones (Phil. Trans. Royal Soc. 1888, p. 69) and also a pair of bones behind the squamosals, postsquamosal bones as they may be named, which are in the position of the bones previously termed epiotic. The preservation in Pareiasaurus of this region of the skull leaves much to be desired, but it suggests comparison with Procolophon. Text-fig. 31. Type specimen of Procolophon trigoniceps, from Donnybrook. For comparison with P. minor. The Postorbital bar.-The preservation of the type of Procolophon trigoniceps (text-fig. 31) is not quite satisfactory, owing to cranial bones having scaled oft* from the frontal region and the postorbital area on the right side. On the left side there appears to be a slight, almost imperceptible linear separation between the postorbital and the squamosal and quadrate bones. It might pass as a condition of fossilization, since it is absent in P. minor, but for the circumstance that the condition becomes a foramen in P. laticeps (text-fig. 32). There is no trace of the slit on the right side of the skull. There the sutural lines indicate a long narrow strip of bone descending |