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University of California Publications in Zoology 



[Vol. 21 



included for purposes of computation in any of the three, regions 

 designated in table 6 — the time of increase would be altered slightly 

 since, in the maxillae, the greatest amount of increase in length 

 occurs between F and J and not between C and E, as is the case with 

 the frontals and palatines. Dorsally, the greatest amount of increase 

 in length in the postorbital region occurs between C and E; but 

 ventrally, in the basisphenoid and basioccipital, the greatest amount 

 of increase occurs between F and J. Tables 2 and 6 present some of 

 the more striking changes. 



TABLE G 



Kblative Amounts of Growth in Length, Depth, and Width in Different 

 Regions of the Cranium and the Ages at which the Increases Occur 



Explanation: i, approximate; dor., dorsal; inm., intermediate; io., least inter- 

 orbital breadth; po., least postorbital breadth; ven., ventral. 



Amounts of increase. — The rostral region, especially on the dorsal 

 surface of the cranium, and the basicranial region increase most. This 

 same feature of development was pointed out in Neoioma by Allen 

 (1894, p. 240) and appears to be a common feature of development in 

 most mammals. The relative increase in length of the rostrum is of 

 course not so great in short-faced forms; for example, Felis (see 

 Jayne, 1898, figs. 399-409). When the increases in the three planes 

 of the skull are averaged in each of the three regions designated, it 

 appears that the greatest relative increase occurs in the preorbital 

 region, the lea.st in the interorbital region, and an intermediate amount 

 in the postorbital region. 



Time of increase. — The age at which the greatest amount of increase 

 occurs in any one of the four age groups appears to be earliest in the 

 postorbital region, latest in the preorbital region, and intermediate in 

 point of time or age, in the interorbital region. 



Certain po.ssible correlations between the amounts, relative degrees, 

 and times of increase with the life-habits of the animal are suggested. 

 Tlie interorbital region in OtospermophU us , during postnatal develop- 

 ment, appears to be the most stable of the three regions discussed above. 



