748 B. W. KUNKEL 



In it;^ relation to the quadratum, the squamosum agrees closely 

 with that established by Thyng ('06) as the criterion of that 

 bone. 



The quadrato-jugale is not present in the stage modelled, but 

 appears first in my next older embryo (carapace length, 13.5 

 mm.). In this it forms a triangular plate with its ventral angle 

 greatly prolonged in front of the pars quadratum of the palato- 

 quadratum. Its postero-dorsal angle lies a short distance ven- 

 tral to the anterior angle of the squamosum and its postero- 

 ventral margin follows closely the free anterior margin of the pars 

 articularis of the palatoquadratum. Its anterior portion comes 

 to lie ventral to the posterior end of the postfrontale and pos- 

 terior to the posterior end of the zygomaticum. In older embryos 

 the posterior end of the quadrato-jugale is overlapped externally 

 by the squamosum. 



It is of special interest in this connection that in such chelonians 

 as Cistudo ornata, Chelodina longicollis, and Geoemyda spin- 

 osa, the embryonic character of the absence of a quadrato-jugale 

 is retained through life. 



The zygomaticum {z., fig. 26) in the stage modelled is in the 

 form of a long, slender plate of bone bent so that it exhibits one 

 limb extending horizontally forward and the other obliquely 

 postero-dorsally. At the same time the plane of the bone is 

 somewhat twisted on its own axis so that whereas the postero- 

 dorsal portion is sagittal, exhibiting a lateral and medial surface, 

 the anterior portion is more nearly horizontal, exhibiting a dorsal 

 and a ventral surface. The bone lies freely in the postorbital 

 region quite far in front of the front margin of the pars quadrata. 

 Together with the postfrontale, it completes the posterior margin 

 of the orbit. Its anterior end extends forward dorsal to the 

 hinder end of the maxillare. 



The maxillare (m.) consists of a thin elongated plate of bone 

 having in general a horizontal position and forming the ventro- 

 lateral margin of the anterior end of the skull. Its dorsal sur- 

 face is slightly concave so that it exhibits a shallow longitudinal 

 groove; ventrally it is strengthened by a vertical rib which grows 

 higher toward the anterior end. The plane of the horizontal 



