288 ZALOPHUS CALIFORNIANUS CALIFORNIAN SEA LION. 



The skull changes greatly in its proportions with age. The 

 capacity of the brain-case does not greatly increase after birth, 

 it enlarging mainly by the thickening of its walls. The ante- 

 rior half of the skull develops rapidly and alters very much in 

 form. At birth the inter- and anteorbital portions of the skull 

 are very short, they together forming rather less than half the 

 length of the skull, while in full-grown skulls they comprise about 

 two-thirds of its length. In a young skull (Nat. Mus., No. 15660 y 

 taken from an animal killed a few days after birth), which has 

 a total length of 146 mm., the brain-case alone has a length of 

 78 mm., the interorbital region* a length of 31 mm., and the ante- 

 orbital a length of 37 mm., giving a total length from the an- 

 terior wall of the brain-case to the front border of the intermax- 

 illa3 of 68 mm. In this skull (as in several others before me of 

 about the same age) the occipital condyles are wholly anterior 

 to the plane of the occiput. In a very old female skull (M. 0. Z. 

 Col., No. 6150), with a total length of 233 mm., the occipital con- 

 dyles project 15 mm. behind the occipital plane. Of the remain- 

 ing 218 mm. of the length, the brain-case occupies 83 mm., the 

 interorbital region 65 mm., and the anteorbital 71 mm., and the 

 two regions together 136 mm. In the first the ratio of the 

 length of the brain-case to that of the rest of the skull is as 78 

 to 68 ; in the last as 83 to 136. In a middle-aged male skull, 

 the total length is 282 mm., of which the coudylar extension is 

 22 mm. Of the remaining 260 mm., the brain-case occupies 95 

 mm., the interorbital region 78 rum., and the anteorbital 87 mm., 

 making the proportionate length of the brain-case to the rest of 

 the skull as 95 to 165. The ratios between the different regions 

 in these three skulls are as follows : 



The width of the brain-case in these skulls is respectively 

 90 mm., 97 mm., and 107 mm. 



In adult skulls the breadth of the interorbital region is rela- 

 tively, and generally absolutely, much less in adult skulls than 

 at birth, and the point of greatest constriction is placed much 



* That is, the narrow portion of the skull bounded laterally by the tempo- 

 ral fossae and orbits. 



