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Show 1905.] PRIMITIVE REPTILE PROCOLOPHON. 219 evident in Trirachodon and other genera. The differences between the two specimens may be found to justify generic separation. The Quadrate Bone.-The most striking difference is in the character of the bone which articulates with the mandible (text-figs. 30, 31). In P. minor the quadrate bone is partly imbedded in matrix, so that there is no reason to suppose that any structure is lost from that region. The quadrate bone is directed downward and backward, is compressed from front to back, forms a transverse articulation, somewhat constricted in the middle, and is thickened on the lateral external surface above the articulation ; but the bone shows no indication of the posterior development which was named squamosal by Sir R. Owen, and afterward regarded as probably quadrato-jugal by myself, which is so well developed inP.trigoniceps (text-fig. 31). A fresh examination of these and other skulls leads me to remark that the place of the quadrato-jugal bone is between the malar bone and the quadrate, but there is no ossification in that position in Procolophon. Therefore I infer that the quadrato-jugal bone has no existence in Procolophon. The thick cellular bone which extends from the jugal behind the articulation I am unable to separate from the quadrate bone, which articulates with the mandible, since no specimen shows a dividing suture between it and the bone which articulates with the mandible. This determination, if sustained, removes the anomaly of the quadrato-jugal attaining an enormous thickness. Its supposed position behind the malar and external to the quadrate was paralleled by the thin quaclrato-jugal in Ichthyosaurus. The Parietal Region.-The region behind the frontal bones and orbits, which is commonly termed parietal, shows faint obscure markings in P. minor (text-fig. 30) of lines in a transverse curve from the bone named epiotic to the hinder border of the parietal foramen, and short longitudinal lines prolonging the inner and outer borders of the orbits backward. The latter led me formerly to suppose that the postfrontal occupies a quadrate area in front of the epiotic extending forward to the orbit. The only other specimen in which the parietal region appears to be divided in similar way by faint markings is the British Museum skull R. 1999. The parietal bone is composite in Mochlorliinus and other genera. But while the appearances in Procolophon may be due to squamous overlap of bones, the evidence is insufficient to establish their nature, though it strongly suggests the structure in some Labyrinthodont skulls. The Postsquamosal Bone.-The bone which is found at the posterior external angle of the flat parietal region I have formerly referred to as the epiotic. It corresponds in position with the bone so named in Labyrinthodonts, though, as most writers on Labyrinthodonts have remarked, it has nothing in common with the otic bone named epiotic by T. H. Huxley. This ossification is named squamosal by Dr. A. S. Woodward in his ‘ Vertebrate Palaeontology,' but it is a thin plate of bone, quite distinct from the squamosal and superimposed upon it. If the |
