10 ODONTORNITIIES. 



and were separated above by an osseous median crest, depending from the 

 roof of the brain case. The optic lobes (figure 1, op) were large, and 

 very prominent, and only slightly covered above by the cerebral hemis- 

 pheres. Their similarity in size and position to the optic lobes of reptiles 

 is especially noteworthy. The optic nerves were large. The cerebellum 

 (figure 1, cb) was quite large, and reptilian in its general features. The 

 floccidi were well developed, and lodged in distinct cavities. 



In contrasting the brain-cavity of Hesperomis with that of Colymbus, as 

 shown together in figures 1 and 2, a striking difference is seen in the size, 

 the latter being about three times the bulk of the former. The two skulls 

 are represented of the same absolute length, for the purpose of direct 

 comparison. If, moreover, the relative size of the entire skeleton in each 

 case be likewise considered, the brain of Hesperomis would have even less 

 than one-third the relative capacity of that of the Loon. As the two birds 

 were evidently similar in shape, and habits, the comparison seems to be a 

 fair one. Another marked difference in the brain of the two genera is seen 

 in the relative size of the cerebral hemispheres, as represented in figures 1 

 and 2. The cerebral lobes of Colymbus are very large, and much expanded 

 transversely ; and it is in this portion of the brain that the real difference 

 of size is most apparent. 



These facts are especially important, since they tend directly to show 

 that the essential principles of the law of brain-growth, established by the 

 writer in extinct mammals, applies also to birds. This law, briefly stated, 

 is as follows : 



1. All Tertiary mammals had small brains. 



2. There was a gradual increase in the size of the brain during this 

 period. 



3. This increase was confined mainly to the cerebral hemispheres, 

 or higher portion of the brain. 



4. In some groups, the convolutions of the brain have gradually 

 become more complicated. 



5. In some, the cerebellum and the olfactory lobes have even 

 diminished in size. 



