390 



Vniversiiy of California Publications in Zoologij 



[Vol. 21 



figure 26 the incus is much nearer the posterior end of ^Meckel 's 

 cartilage than is the malleus. 



Teeth. — The dental formula of Otospermophihis is: \, ^, \, |. 

 The incisors both above and below appear in D. The fourth premolars 

 both above and below appear in F. Pm- appears in G. The first 

 molars appear in G but Mj is farther advanced than M-. The second 

 molars are present in //, M2 again being better developed than M-. 

 M3 appears in 1. M- appears later but is well developed in J. The 

 deciduous upper premolars are both replaced between K and L. The 

 lower permanent premolar does not appear until N. It is seen, then, 

 that the upper and lower incisors and milk premolars appear at 



42 



mec. r 

 43 



Figs. 40-43. Buccal and crown views of unworn right upper premolars, X 0. 

 Fig. 40, i, permanent Pms^-. Fig. 41, R, milk Pm-. Fig. 42, L, permanent 

 Pms2-4. Fig. 43, H, milk Pms^^. 



Explanation: ac, anterior cingulum; me., metacone; mec, metaconule; pa., 

 paraoone; pr., protocone. Terms used are for convenience of reference and are 

 not intended to imply homologies. 



approximately the same time ; the lower molars and the upper per- 

 manent premolars appear earlier than their respective opposites. The 

 crown surface of permanent Pnij presents no striking differences from 

 the crown surface of the milk tooth either in size or in number and 

 arrangement of the conids. Pm^ in both the milk and permanent 

 dentition is spike-like and of little or no functional value ; the milk 

 tooth is only two-thirds the diameter of the permanent tooth. The 

 roots of milk Pm- are more widely spread than are the roots of the 

 corresponding tooth in the permanent dentition. Pm- of the milk 

 dentition is at first set squarely in the maxilla but, as the permanent 

 tooth develops, the preorbital portion of the skull lengthens and the 

 anterior root of Pm- is carried forward. This appears to be a mechan- 

 ical adaptation for forcing the anterior root of Pm- out of its socket, 

 thus raising the tooth. The following differences are to be noted 

 between unworn Pm- of the milk and permanent dentitions: The milk 

 tooth is 0.1 mm. shorter, 0.3 mm. narrower, and more triangular in 



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