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 sti'ucture 

 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 in F. trigoniceps 

 (text-fig. 31). A fresh examination of these and other skulls leads 

 me to remark that the place of the quadi-ato- jiigal bone is between 

 the malar bone and the quadrate, but there is no ossification in tha,t 

 position in Frocolophon. Therefore I infer that the quadrato-jugal 

 bone has no existence in Procolojjhon. 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 deter- 

 mination, 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 

 quadrato-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 Mochlorhinus and 

 other genera. But while the appearances in Procolophon may be 

 due to sqviamous overlap of bones, the evidence is insulficient to 

 establish their nature, though it strongly suggests the structure 

 in some Labyrintliodont skulls. 



The Postsquamosal Bone. — The bone which is found at the 

 posterior external angle of the fiat 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 Palfeontology,' but it is a thin plate of bone, quite 

 distinct from the squamosal and superimposed upon it. If the 



