8 THE MORPHOLOGY OF THE SKULL 



squamosal would come in contact, and a union of the two would produce the bone 

 called squamosal or squamosal + prosquamosal in Sphenodoii. 



The prosquamosal: The prosquamosal has the position usually assigned^ to the 

 quadrato-jugal ; that is, it connects the jugal and the quadrate. It would have been 

 taken for the quadrato-jugal in the present specimens if the presence of the foramen 

 quadratuvi had not indicated the true position of the quadrato-jugal. (The significance 

 of the position of the prosquamosal is discussed in the description of the temporal 

 region below.) The prosquamosal joins the jugal in about the middle of the inferior 

 temporal arch, the tAvo bones narrowing somewhat as they approach, so the edges of 

 the inferior arch are concave both above and below. Posteriorly the prosquamosal 

 widens, so that it has an upper and lower process and the bone becomes roughly 

 T-shaped. The lower three quarters of the posterior edge join the quadrato-jugal and 

 the upper quarter joins the anterior edge of the posterior process of the postorbital to 

 form the posterior edge of the superior temporal vacuity. There is a little doubt as to 

 whether the prosquamosal joins the edge of the quadrato-jugal directly or passes under 

 it, articulating with the lower surface, and finally articulates with the edge of the 

 quadrate near the quadrato-jugal. The specimen No. 1002 seems to indicate the 

 latter condition on one side. 



The bones forming the edges of the superior temporal vacuity are approximated 

 so the vacuity is very small. 



In the crushed specimens the sides of the upper vacuity are very close together 

 and it seems that they must have been so in life. The edges of the bones where they 

 Avould meet are very thin and it is possible that they did meet over the vacuity in 

 specimen 1001, although there could have been no articulation even in this case. It 

 is impossible to say positively whether this is an appearing or a disappearing vacuity 

 but the former seems to be the most probable from all considerations. In IHopeus 

 the most primitive member of the Clepsydropidas, the superior vacuity is very small or 

 absent. In specimen 1001 there is a strong rugosity of the lower ends of the parietal 

 which covers the vacuity but this I am inclined to regard as pathological. 



From the foregoing it will be seen that so far from the quadi-ate region of the 

 skull being depressed and approaching the Theriodonf type with any relation to the 

 development of the mammalian skull it is elevated and of the most primitive char- 

 acter and in connection with all the other specializations of the skeleton of the Ameri- 

 can Fclycosauria {Clepsiidropidpc) indicates rather the approaching culmination of a 

 side branch of the primitive stem than the true progress of the Sauro-mammalian mu- 

 tation which was seemingly accomplished in Africa. It is not proven however, as 

 Osborn suggests, that the Gomphodontia were descended from forms with primitively 



