Ti'KNKi!, J/c)/-y>//()A>4'r of t)ic filial/ /h-aii/. | :^ 



3. A convex curve, w^hich extends caudo-laterad from the 

 caudad extremity of the second curve to the caudad extremity 

 of the hemisphere. 



4. A convex curve, which extends cephalo-laterad from 

 the caudad extremity of the third curve to the widest part of 

 the prosencephalon, then cephalo-mesad to the cephalic ex- 

 tremity of the same. The widest part of the prosencephalon 

 is nearer its caudad than its cephalad extremity. 



This is the usual appearance; but, in some types [Ai-- 

 deidce, Cuculidce, Meleagrididce ^ Domestic Fo-wl^ Ttirdidce) , 

 the caudad and cephalad portions of the fourth curve are 

 convex, while the middle portion is concave (Plate V, Figs. 

 6, 8). Each of the above curves gi-ades so smoothly into its 

 successor that it is impossible to say where one curve ends 

 and another begins. 



When the two hemispheres are in their natural position, 

 their combined outlines form a V at each extremity of the 

 prosencephalon. The smaller, cephalic V, is formed by the 

 intersection of curve number one of one hemisphere with 

 the corresponding curve of the other hemisphere. The 

 larger, caudal V, is formed by the intersection of curve 

 number three of one hemisphere with the corresponding 

 curve of the other hemisphere. 



In the Anatidiv (Plate V, Fig. 5) and Ardeidce (Plate V, 

 Fig. 6) the caudal V is very shallow. As we ascend the 

 scale, this V becomes deeper and deeper (Plate V, Figs. 

 8, 10). This I consider an affirmation of the above theory; 

 that, in its evolution, the lateral border of each hemisphere 

 extends caudad much more rapidly than the mesal. This 

 causes the caudal border of each hemisphere to revolve 

 towards the meson. The depth of the caudal V is a function 

 of this revolution, and varies directly as the amount of revo- 

 lution. According to this theory, the caudal V should be 

 shallow in the lower and deep in the higher types of avian 

 brains. Since this is the cases, I think we have an import- 

 ant confirmation of the truth of the theory. 



