1926] HaU: Shull of the Rodent OiospermopMus grammurus beecheyi 393 



of zygomatic processes, 176; breadth of rostrum, 174; least postorbital 

 breadth, 127; Pm. m. tooth row, 123; width of parietals (dorsal), 

 110; palatal width at alveoli of first molars, 90. Of the regions 

 measured the percentage of increase is least for the palatal width at 

 the alveoli of the first molars ; thus it represents the most stable portion 

 of the skull, of which measurements were taken. In fact, as will be 

 seen from table 2, this measurement is greater in F and in a number 

 of succeeding skulls than it is in Q. The mea.surement of the length 

 of the nasals shows the greatest amount of increase, 370 per cent. 

 This, then, appears to be an unstable region of the skull. A priori, a 

 relatively well developed condition of those parts of the skull which 

 increase most during postnatal development would seem not to be so 

 essential to the welfare of the animal at birth as would a relatively 

 well developed condition of the parts which increase least after birth. 



The five ages represented in table 5 were chosen on a basis of roughly 

 equal amounts of increase in the occipito-nasal length between any 

 two ages. Similar measurenients of the other skulls are intermediate 

 between those here given, and, except in a few cases, fall in the 

 expected places. The only instance of a deviation from the average 

 measurement of the age represented sufficient seriously to affect the 

 computations given in table 5 is that of the palatines in J. This 

 accounts for the absence of an increase between F and J. From G to K 

 the palatal length is as follows: 6.4, 6.4, 6.3, 6.1, 6.8. In table 5 all 

 measurements of length were taken along the sagittal plane of the skull 

 except that of the palatines. This measurement was taken slightly obli- 

 quely because greater accuracy was obtained in this way than by meas- 

 uring on the median line, which required the inclusion of the palatal 

 spine. The palatal spine was found to vary greatly in length, irrespec- 

 tive of age, and was often destroyed. The per cent of increase given 

 after the amount of increase that occurs between any measurement of 

 one skull and the same measurement of a preceding skull is the per cent 

 of the total amount of increase. For example, there is a total increase 

 of 43.5 mm. in the occipito-nasal length. This represents 100 per cent. 

 Of the total increase of 43.5 mm., 11.3 mm. is made between C and E. 

 This equals 26 per cent of the total 100 per cent. 



Tables 2 and 5 show an increase in the occipito-nasal length of 250 

 per cent.^ Of the component elements contributing to this increase, 



1 The occipito-nasal length measurement and all other measurements given 

 in this paper are straight measurements. The measurements of the component 

 parts of the dorsal surface of the skull, when added together, give a curved 

 measurement. Thus, a discrepancy between the sum of the measurements of the 

 component parts of the dorsal surface of the skull and occipito-nasal length, 

 aside from that of probable error, exists here. 



