358 TJniversiiy of Califomhi PiihJicatioiis in Zoolof/n [Vol. 21 



Individual Skull Elements 



Nasals. — Tn A each nasal is represented by an ossification that is in 

 contact laterally with the ossified premaxilla and medially with the 

 other nasal. Posteriorly the nasals are separated from the frontals 

 by a fontanelle that is bridged in B. In C the nasals are slightly nar- 

 rower posteriorly than anteriorly and are still narrower in D. The 

 posterior constriction in C and D results from the lateral border of 

 each nasal bending slightly but sharply mesially and continuing in a 

 straight line until it reaches the truncated posterior border. In E the 

 posterior half of the lateral margin becomes bent mesially near its 

 middle. This median convexity of the posterior portion of the lateral 

 border of each nasal and the constriction of the whole posterior four- 

 fifths of the nasals become gradually accentuated from E through the 

 successively older specimens. As will be seen from table 2, the nasals 

 become relatively (to their width) longer in the skulls successively 

 older than E. In most of the skulls of Q the nasals are narrowest not 

 at their posterior end but at a point 1 to 3 millimeters anterior to the 

 posterior end. The transverse depression of the nasals, which is 

 situated about one-third their total length from the anterior end, 

 appears first in E, and in F is as well developed as in any of the later 

 stages. During postnatal development the nasals increase more than 

 3i/> times in length and they double in width (see table 2). 



Premaxillae. — The premaxillae in A are thick ossifications in con- 

 tact dorsolaterally and laterally with the nasals, and posteriorly both 

 ventrally and laterality with the maxillae. No ossification exists 

 between the anterior palatine foramina. The region of each posterior 

 wing of the premaxillae, that later extends posteriorly between the 

 maxilla and nasal and overlaps the frontal, is not yet ossified. In B 

 ossification is complete between the anterior palatine foramina but aii 

 unossified area still exists between the dorsal wing of the premaxilla 

 and the frontal. In C the inner part of the wing that lies beside the 

 nasal is ossified and is in contact with the frontal. A fontanelle 

 bounded by the frontal, lacrymal, maxilla, and premaxilla persists 

 on each side in specimens as old as E. The same fontanelles, but with 

 the lacrymals excluded from their borders, persist as late as G (see 

 fig. 10). Of the several fontanelles these are the last ones to disappear. 

 Figure 20 shows the incisor alveoli occupying a large area on the 

 ventral surfaces of the premaxillae in A. Here the alveoli lie partly 

 posterior and mostly lateral to the anterior palatine foramina. The 



