394 University of California Publications in Zoology [Vol. 21 



the nasals are chiefly responsible. As measured on the dorsal surface 

 the parietals plus the interparietal increase less than the frontals or 

 the nasals. The parietals, and especially the interparietal, are more 

 important elements in roofing the cranial cavity in the late embryonic 

 stages of growth than are the frontals (see figures of the young skulls). 

 The frontals gradually play a more and more important role with 

 increase in age. 



Table 5 shows that a greater- increase in the occipito-nasal length 

 occurs between F and J than in any of the other three stages. Of the 

 bones forming the roof of the skull only the nasals show the greatest 

 amount of increase at this time. The frontals and parietals plus inter- 

 parietal show the greatest increase between C and E. This again 

 indicates the more important part played by the nasals than by the 

 frontals or by the parietals plus interparietal in determining both the 

 amount of increase in the occipito-nasal length and the lime at which 

 the occipito-nasal increase occurs. The relatively great increase of 

 the nasals is indicative of the large amount of growth that occurs in 

 the rostrum as a whole. This relatively great increase in length of the 

 rostrum begins at the time the animal commences to take solid food. 

 Thus one would expect to find the largest increase in length of the 

 premaxillae occurring between F and J, and table 5 shows this to be the 

 time at which it does occur. This disproportionate growth of the 

 rostrum and, in fact, of the whole preorbital portion of the skull, con- 

 tributes as much as any other one factor to altering the appearance and 

 general shape of the skull. 



Except the palatines, all other elements on the ventral surface of 

 the skull increase most in length between F and J (see table 5). The 

 greatest increase in the palatines occurs between C and E. The 

 maxillae rank first in the amount contributed to the increa.se in length 

 of the entire condylo-basal extent of the skull, the basioccipital second, 

 and the premaxillae third. Again excepting the palatines, the maxillae, 

 while actually increasing more along the midline than any of the other 

 elements have the smallest per cent of increase. In greatest per cent 

 of increase the basisphenoid is first, the basioccipital second, and the 

 premaxillae, third. The small per cent of increase of the palatines and 

 of the maxillae accounts for an increase of only 244 per cent in the 

 condylo-basal length. Whereas the overlapping of the dorsal skull 

 elements masks the actual amount of growth in the elements tliat are 



I 



- Gresiter or greatest is used relative to the larger or largest amount occur- 

 ring in any one of the four periods in which increases are computed. For these 

 periods see table 5 or 6. 



