1926] HaU: Skull of the Modent Otospermopliilus grammiinis beeclieyi 377 



of the same age as F, the presphenoid is widest posteriorly. In the 

 specimen slightly older than M the presphenoid is 10 mm. long, 0.9 mm. 

 wide anteriorly, and 2.6 mm. wide posteriorly. On its dorsal surface, 

 between the two optic foramina, a fossa is present that partly 

 accommodates the swollen region of the brain in which the optic 

 chiasma lies. 



Basispkcnoid. — A small transverse elliptical ossification represents 

 this bone in A. At this stage the ossification is in contact with no 

 other bone, but in B it has reached the alisphenoids, basioccipital, 

 palatines, and pterygoids. The presphenoid and basisphenoid meet 

 in F. In Q the basisphenoid is fused with all the bones mentioned 

 above with which it was in contact before E. It also establishes con- 

 tact with the fused margins of the petrosals and tympanies by a 

 process that is sent out on either side. On either side one of these 

 processes separates the foramen ovale from another foramen just 

 posterior to it. Posteriorly, in all stages from C on, the basisphenoid 

 abuts squarely against the anterior end of the basioccipital. No 

 examples in which the two bones were fused were seen. The hypo- 

 physeal fossa on the superior face of the basisphenoid is slightly 

 anterior to the center. The fossa is about 2 mm. across and 1 mm. 

 deep. No clinoid processes are present. In C the basisphenoid is 

 2 mm. long. A small canal that carries a blood vessel extends trans- 

 versely through this bone slightly back of the center. In a skull from 

 the same litter as F, the basisphenoid is 3.2 mm. wide anteriorly, 

 3.9 mm. wide posteriorly, and 5.4 mm. long. In a skull slightly older 

 than M, the measurements in millimeters are : width anteriorly, 3.-4 ; 

 width posteriorly, 6.0 ; length, 8.8. In Q this bone averages 10.5 mm. 

 in length. 



Basioccipital. — Figures 3 5-20 show the changes in shape. One 

 difference in the manner of development of the basioccipital and of 

 the basisphenoid may be seen in figure 20. The ossification in the 

 basisphenoid is elliptical, and the long axis lies transversely with 

 respect to the long axis of the skull. The elliptical oval area of 

 ossification in the basioccipital has its long axis extending longitudin- 

 ally Math respect to the long axis of the skull. At this time, A, the 

 area of ossification of the basioccipital is more than twice that of the 

 basisphenoid, while the length of the basioccipital is always 1 to 2 mm. 

 greater than that of the basisphenoid. A median ridge is present 

 on the inferior surface of the basioccipital in adults. Traces of this 

 appear first in G. Two large fossae develop at the same time that this 



