1926] HaJl: SJatll of the Eodent OtospermophUus grammurus heecJieiti 357 



MATERIALS 



Approximately 600 separate skulls, 175 skulls-with-skins, and 25 

 litters of embryos of the California Ground Squirrel have been avail- 

 able for study ; nearly all of this material is contained in the California 

 Museum of Vertebrate Zoology. In dealing with changes that occur 

 with growth, only males have, as a rule, been used. The exceptions 

 were specimens nos. 34289 and 34292, Mus. Vert. Zool., which are 

 females of the exact ages of which no males were available. The 

 measurements given of these two young skulls would not, it is believed, 

 differ from the same measurements taken of male skulls of the same 

 age, since comparisons of male and female skulls slightly older showed 

 no differences due to sex. 



Considerable pains were taken in selecting the male skulls believed 

 to be typical of different ages. The entire series was arranged in 

 probable order of age from youngest to oldest, using as criteria the 

 degree of development of the various skull elements. Measurements 

 taken to 0.1 millimeter were freely used in selecting specimens most 

 typical in all respects of the different ages. Table 2 gives certain 

 cranial measurements, ratios of these measurements to occipito-nasal 

 length, and amounts of increase and per cents of increase for each 

 measurement, of 14 males selected in the above manner as well as for 

 2 females and for the average of 8 very old males. For convenience of 

 text reference, letters A to Q are used for these skulls in place of the 

 Museum catalogue numbers. All catalogue numbers given in this table 

 and elsewhere in the text refer to specimens in the collection of the 

 Museum of Vertebrate Zoology. A represents the skull of a 40 mm. 

 embryo and i? of a 44 mm. embryo. C represents the skull of an animal 

 found dead 6 days after birth, in the nest of a litter of young born in 

 captivity. It was badly decomposed and represents a stage at birth or 

 within a very few days thereof. D stands for the skull of an animal 

 7 days old, and E of an animal 15 days old. N, and those designated 

 by preceding letters, represent skulls of animals less than one year 

 old. 0, P and the series of skulls of 8 very old males designated as Q 

 refer to skulls of animals more than one year old. 



The ratios for the different measurements are the ratios to the 

 occipito-nasal lengths. For example, to obtain the ratio of the palatal 

 length of A the following equation is made and solved for x, which in 

 this case is found to be 56.3. 11.2 {occipito-nasal length) : 100 :: 6.3 

 (palatal length) : x. 



