384 Universiiy of California Puhlications in Zoologi/ [Vol.21 



posed between the parietals immediately in front of the interparietal. 

 Unlike Erethizon, Castor, and Marmota, Otospermophilns (jranunurus 

 beecheyi has few Wormian or fontanelle bones. Aside from the 

 possible occurrence in C only one small Wormian bone was noted in 

 the approximately 800 skulls of this form that I examined. This series 

 of 800 skulls represents animals ranging in age from small embrj^os to 

 very old individuals. The Wormian bone noted occurs in /. Its shape, 

 size and position are shown in figure 3. 



Petrosals. — The petrous portion is well ossified in E. The mastoid 

 portion, in F, is represented only by a small ossification less than 

 1 sq. mm. in area. It is fully ossified in G. The lateral wall of the 

 parafloccular fossa is the last part of the mastoid to ossify. Externally, 

 in H, the mastoid appears as a square bone surrounded by the squa- 

 mosal, temporal, parietal, supraoccipital, exoccipital and tympanic. 

 Gradually, however, the squamosal encroaches upon the mastoid and 

 in Q leaves only a small roughly triangular portion of it exposed. The 

 base of the triangle bears the mastoid process. The mastoid fuses with 

 the petrous at an early age. The two are not clearly separated from 

 each other by suture in F. It will be recalled that at this time the 

 mastoid portion is not fully ossified. Internally the mastoid is per- 

 forated by the large aperture of the parafloccular fossa. Below the 

 anterior half of this aperture, on the petrous, there is a depression 

 into which the internal auditory meatus and a second canal, probably 

 the facial, open. In a stage slightly younger than A three clearly 

 defined parts of the petrosal can be recognized. The anterodorsal, or 

 squamosal portion, soon fuses with the underlying petrous portion 

 proper. At the stage slightly earlier than A, referred to above, the 

 squamosal portion bears a downward facing shelf against which the 

 posterior end of Meckel's cartilage abuts. Later, a cup-shaped 

 depression or socket develops at this place to receive the posterior end 

 of Meckel's cartilage. A striking feature of the petrous is the develop- 

 ment of a bar that extends forward and inward over the basisphenoid 

 (see fig. 27). The two bars, one from each petrosal bone, meet at the 

 median line of the skull and in adults are fused with each other at this 

 point. The bars are closely applied to the floor of the brain case 

 except where the two meet, which is at the posterior margin of the 

 h^ypophyseal fossa. Here their anterior margins are raised slightly 

 and apparently overlap part of the hypophysis. A process of each bar 

 extends a short distance forward along either side of the hypophysis. 

 Since true clinoid processes are absent it would seem from the position 

 and shape of these bars that they subserve the function that is usually 



