1926] Hall: Skull of the Eodent Otospermophilus grammurus heecltei/i 371 



occipital ; both tlie parietal and squamosal have their posterior marg-ins 

 reflexed upward to participate in the formation of the lambdoidal 

 crest. Up to and including / the outward projection of the zygomatic 

 portion of the squamosal proceeds slowly, but after this time the curved 

 portion of the squamosal is rapidly straightened. 



Jugals. — In A each jugal is ossified and reaches from the zygomatic 

 process of the squamosal to the lacrymal region. It extends along the 

 inner side of the maxilla. In C the inner lip of the zygomatic portion 

 of the maxilla that bounds the anterior part of the jugal has grown 

 halfway up the side of the jugal. In E this part of the maxilla is pro- 

 portionally as far extended over the mesial surface of the jugal as it is 

 in Q. In Z> the zj-gomatic arch is bowed inward at a point near the 

 middle of the jugal bone and remains so up to and including G. In H 

 and L it is straight. From this time on it gradually becomes bowed 

 outward to a greater extent. The jugal bone increases in length from 

 6.5 mm. in C to 26.7 mm. in Q. In width it increases from 0.6 mm. 

 in C to 4.7 mm. in Q — an increase of 311 per cent in length and 683 

 per cent in width. Although the bone becomes very Avide, in Q it is 

 only 0.5 mm. in thickness, or 0.2 mm. more than in C. The main 

 changes are an increase in width and a bowing outward. The jugal is 

 also modified by muscles that attach to it. In Q a distinct lip, and also 

 a transverse ridge, are developed on the dorsal margin of the jugal just 

 back of its union with the outer slip of the zygomatic process of the 

 maxilla. The position of the ridge relative to the length of the skull 

 is modified by the central and posterior portions of the bone being 

 moved relatively farther from the walls of the cranial cavity. This is 

 partly accomplished by the bowing outward of the jugal but is more 

 directly due to the straightening of the basal portion of the zygomatic 

 process of the squamosal and to the development of a progressively 

 wider angle between the longitudinal axis of the skull and the zygo- 

 matic process of the maxilla. This causes the skull to be relatively 

 wider in adults than in young specimens. For successively older skulls 

 this increase is shown in table 2 by the increasing ratio of the zygomatic 

 breadth. It is also readily perceived without the aid of measurements 

 from figures 1-6 and 15-20. 



Orhitosphenoids. — Of the several bones represented within the 

 orbit the orhitosphenoids are the last to ossify. The first ossifications 

 appear in B on the anterior, ventral, and posterior sides of the optic 

 foramina and are in contact only with the palatines. In E ossifications 

 extend entirely around the optic foramina, and most of the posterior 



