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After examining- a large mimlier of bird brains, it was 

 decided to divide the carinate l)irds into two major groups. 

 In the first group, which has been called " A," the greatest 

 width of the prosencepalon is more than 90 per cent, of the 

 length of the brain. In the second group, which has been 

 called " B,'' the greatest width of the prosencephalon is less 

 than 90 per cent, of the length of the brain. The waders 

 and fowls and their affines are placed in group " A." The 

 remainder of the carinate birds fall in group " B.'' 



From the first appearance of the class Aves until now, 

 there has been a gradual retrograde evolution of the avian 

 rhinencephalon. In the lowest types, the rhinencephalon is 

 terminal and is composed of two distinct lobes; in the 

 highest types, the rhinencephalon is not only not terminal, 

 but it is covered by the prosencephalon and partly imbedded 

 in it. In these types, the two lobes of the primitive rhinen- 

 cephalon have fused to form a small single lobe. 



In the light of these facts, group -'A "has been sub- 

 divided into two minor groups, "a" and " b." In those 

 brains that fall into group " a," the partly imbedded rhinen- 

 cephalon is entirely covered by the prosencephalon and is 

 composed of a single lobe. In those brains that fall into 

 group " b," the partly imbedded rhinencephalon is covered, 

 but it is composed of two lobes. 



Upon the same principle, group "B" has been sub- 

 divided into two minor groups, " c " and " d." In those 

 brains that fall into group " c," the rhinencephalon is com- 

 posed of two lobes and is sub-terminal. In those brains that 

 fall into group " b," the rhinencephalon is composed of two 

 lobes and is terminal. 



These minor groups have been redivided into two groups. 

 In this case the position of the optic lobes has been the 

 criterion. Division " I '" includes those birds in which the 

 optic lobes are entirely covered by the prosencephalon; 

 division " II " contains those in which the optic lobes are 

 only partially covered. 



