1926] Hall: ShuU of the Rodent Otospermophilus grammuriis ieecheyi 401 



10. In postnatal development the preorbital portion of the skull 

 undergoes a greater amount of growth than do either the interorbital 

 or postorbital portions. The interorbital portion undergoes the 

 smallest amount of growth and the postorbital portion an intermediate 

 amount. 



Study of a Large Series 



A series of 445 skulls from western Contra Costa County, Cali- 

 fornia, all collected in September, 1918, were available for examina- 

 tion. In a series of this size, collected at one locality in one month 

 of the same year, one might expect to find a number of definable age 

 groups equal to the number of years of life" which individuals of the 

 species attain. Two groups including animals less than one year old 

 and animals more than one j-ear old ca)t be determined and groups 

 of skulls corresponding to age in years of animals more than one year 

 old are indicated, but the comparative paucity of material of the stages 

 assumed to be more than one year old renders separation into groups 

 corresponding to years of age rather hazardous. Even betAveen the 

 groups designated as less than and more than one year old there are 

 intermediates. However, by taking account of the large number which 

 cranial characters indicate to be of the same general age, separation of 

 the two groups can be made with considerable assurance. The reasons 

 why a grouping by years of age cannot be made for animals more than 

 one year old are : insufficient material, variation of probably five 

 months in age of animals of the same year (see McCoy, 1912, p. 1070), 

 individual variation, decrea.ses in rapidity of changes in the skull with 

 increase in age, and effect of a variable period of aestivation. 



Since shooting was the method generally employed in getting the 

 445 squirrels, the greater wariness of the animals more than one year 

 old might appear to explain the taking of the relatively large number 

 of animals less than one year old. However, the fact that the ratio of 

 adults to young increased as the breeding season approached (see 

 Grinnell and Dixon, 1918, p. 632) indicates that the shooting played, 

 at most, a minor role as a selective factor in this respect. 



Table 7 gives the number referred to each group. From the little 

 that is known of the longevity of this animal, it would appear, figuring 

 the relative number of young and old that are probably alive in Sep- 

 tember, that the young remain above ground in proportionately greater 



3 P. Chalmers Mitchell (1911, p. 446) records a specimen of Spermnphilus 

 firammnrus that lived 9 years and 11 months in the gardens of the London 

 Zoological iSociety. 



