loo Journal of Comparative Neurology. 



developed upon the site of the switch station for the motor 

 peduncles. 



It will be instructive to compare the above with similar 

 sections of a box turtle {Cistudo). 



Figs. 1—9, Plate X, give a clear idea of the topographical 

 relations of the parts and Fig. 10, of the same plate, illustrates 

 the normal structure of the tectum opticum, which may with 

 advantage be compared with the corresponding section of the 

 tectum of the fish shown in Fig. i, Plate VII of the May num- 

 ber of this Journal for 1892. The resemblance is quite unmis- 

 takable though the drawing of the fish is from a transverse sec- 

 tion while that of the turtle is from a horizontal one. 



Fig. I shows the upper part of the habena with the posi- 

 tion of the epiphysis behind and the paraphysis cephalad of the 

 supracommissure. It is especially easy in this type to trace 

 the supracommissural tract. The habena consists of very large 

 cells while Meynert's nidulus cells are scarcely half as large. 

 Fig. 2 indicates the conspicuous isolation of the diencephalon 

 from the sharply elevated optic lobes. 



These sections incline cephalad so that we rapidly en- 

 counter the ventral structures. Another result is that we fol- 

 low a large part of the optic brachia in a single section. In 

 Fig. 3 one can easily distinguish the finer fibres of the brachia 

 from the coarser optic tracts. 



In the midst of the thalamus, immediately below the habena, 

 is a large mass of densely stained material of the exact structure 

 of the ruber but, of course, too far dorsad and cephalad. 

 Whether to regard this mass as a special development of the 

 subthalamicus or an organ peculiar to turtles is at first puzzling. 

 It occurs in the same position in CJielJiydra. Closer inspection 

 reveals a small nidulus of the same general character ventrad of 

 the habena of the black snake. 



A series of longitudal sections may ne.xt be appealed to, 

 the black snake being chosen in this case also. A section near the 

 lateral margin shows the strong postpeduncular tract of the 

 cerebellum wrapped about the lateral aspect of the medulla, 

 arching cephalo-ventrad of the exit of the trigeminus and curv 



