1926J Ball: Skull of tJie Rodent OtospermopJiilus f/ramiinn'tis hecclieiii 3il.") 



overlapped, thus rendering it difficult to define exactly the amounts 

 contributed to the oecipito-nasal increase by the dififerent elements, 

 the existence of a bony palate below the floor of the cranial cavity 

 permits of a sliding motion of the former over the latter during growth 

 and likewise renders an exact statement regarding the respective parts 

 played by the ventral elements, in contributing to the increase in 

 condylo-basal length, difficult to make. Not only do the greatest 

 amounts of increase in different elements occur at different times but 

 the smallest amounts of increase in different elements occur in different 

 ones of the four periods of growth shown in table 5. A similar state- 

 ment, therefore, may be made with respect to intermediate amounts 

 of increase. 



Table 5 shows that the amount and the time of greatest amount of 

 increase are similar in the occipito-nasal and condylo-basal lengths. 

 The ratios of the condylo-basal lengths given in table 2 show, however, 

 that the increases are not exactly comparable. The ratio of the 

 condylo-basal length increases from 93.8 in A to 102.0 in D. It is only 

 in D that the condylo-basal length is less than the occipito-nasal length. 

 From D on, a sharp decline occurs that reaches 90 in G, the lowest 

 ratio in the series. From G to Q the increase is irregular, the fluctua- 

 tions being due, it is thought, to age rather than to individual variation. 



In. width the greatest per cent of increase appears to be made in 

 the mastoid region. The measurement, however, is rather inaccurate 

 since the mastoid ossifies very late, so that its width has to be estimated 

 in the young skulls that have been dried. The other width measure- 

 ments are, with respect to total per cent of increase, from least to most, 

 arranged as follows: least postorbital breath, breadth of rostrum, 

 maximum width of brain case at bases of zygomatic processes, and least 

 interorbital breadth (see table 5). 



In depth, table 5 shows that the greatest total per cent of increase 

 is made in the preorbital region, the smallest total per cent in the 

 interorbital region, while the postorbital portion, measured at the 

 anterior border of the basioccipital, is intermediate. 



The cranium may be divided transversely into three regions, the 

 preorbital, interorbital, and postorbital. The increase in these regions 

 in three planes of the cranium, with respect to time, relative amount, 

 and per cent, is shown in table 6. In constructing this table (6) both 

 the ventral and dorsal elements were considered in determining the 

 relative amounts of increase in the three different regions. If the 

 maxillae had been included in the interorbital region — they were not 



