370 University of California Puhlicafions in Zoolnpii [V^ol. 21 



mosal. The interparietal overlaps the parietal. A firm contact is 

 established between the supraoccipital and parietals. The supra- 

 occipital gTaduallv extends both under and np behind the parietals in 

 successively older skulls. Contact of the i^arietals with the mastoid is 

 gained by the parietal sending a process down beneath the squamosal 

 to meet the mastoid. This process is relatively heavy and forms a 

 strong support for the periotic and tympanic. The mastoids and 

 parietals first come into contact in G. The parietal and parieto-inter- 

 parietal sutures both disappear in L and the fronto-parietal suture in 

 M. Figure 21 illustrates the contact relations of the bones along the 



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Fig. 21. A diagrammatic representation of tlie contact relations of the 

 bones on the dorsal .surface of the skull along the sagittal plane in skulls older 

 than /. 



median line of the skull. The parietal ridges begin to develop in M 

 at which time tAvo diverging lines extend from the postorbital processes 

 back along the sides of the skull, bend inward to meet the short sagittal 

 crest, and gradually develop into ridges. The ridges converge fartlier 

 anteriorly in progressively older skulls. This causes the sagittal crest 

 to become longer. The ridges also become straighter. Figures 1-2 rep- 

 resent two stages in the development of these crests. The lambdoidal 

 crest is also partly composed of the parietals. As the supraoccipital 

 grows upward, it causes the posterior borders of the parietals to be 

 reflected upward. Internally, the parietals are smooth except for a 

 depression that extends along the region of the median suture. 

 Impressions of a few blood vessels are also present. 



Squamosals. — The zygomatic process of each squamosal and the 

 small region from which this process arises are ossified in A. A gradual 

 extension of this ossification carries the squamosal out over the other 

 bones with which it has contact. Each squamosal comes into contact 

 with a jugal and parietal in A, Math an alisphenoid and tympanic in B, 

 with a mastoid in F, and with the supraoccipital in G. The projection 

 outward of the zygomatic process of the squamosal and the overlapping 

 bv this bone of the other bones with which it comes into contact are 

 the main features to be noted in its development. Its relation to the 

 supraoccipital is much like that of the parietal's relation to the supra- 



