46 yoiirnnl of Comparative Neurology and Psychology. 



lowed so distinctly to its end in Galeus as in the teleosts, where all 

 the tracts are more compact. Edinger, who considers it more a 

 decussation than a commissure, saw its fibers end under the tec- 

 tum opticum, while Catois describes it as a commissure between 

 the posterior and lateral regions of the mesencephalon. Haller, 

 who calls it "commissura post-optica superior," says that it 

 contains only decussating fibers of the tecti optici. I do not think 

 that they are tectal decussating fibers (Haller), but rather sub- 

 tectal ones (Edinger) and consider them to be the homologue of 

 the com. transversa of the teleosts and not the homologue of the 

 commissure of Herrick of those fishes. As Edinger has already 

 observed, almost the whole commissure is to be seen in one sec- 

 tion, of which Fig. Ix is a good example. I could not follow them 

 any farther backward, certainly not as far as the extreme posterior 

 part of the mesencephalon, and mdeed it seems very improbable 

 that they should go so far backward. It seems to me that 

 their place of termination is situated in the cell layer under the 

 ventriculus opticus which is to be considered as homologous with 

 the nucleus lentiformis and corticalis of the teleosts and is con- 

 tinued ventrally into the region in which in Gadus I described the 

 nucleus praerotundus. Neither in Galeus nor in Angelus squatina 

 can there be any sharp limitation of the boundaries of these groups 

 of cells. It is only by means of the course of the tracts that the 

 homologies of these regions can be determined. 



Fortunately the relations in the epithalamus are more distinct. 

 Of the ganglia hahenulce I have to say, in the first place, that they 

 seem to be more developed in the selachians than in the teleosts; 

 at least in Galeus canis they are much larger than in the cod, a 

 fish of about the same size. In Fig. liv, Plate III, the situation 

 of both the ganglia, the anterior or lateral, and the posterior or 

 medial, is to be seen. In the posterior part of the whole complex 

 runs the commissura hahenularis, consisting of two kinds of fibers, 

 some medullated and the others unmedullated. The medullated 

 fibers take their origin from the tr. olfacto-hahenularis, which con- 

 tains a rather considerable quantity of myelin and ends partly 

 in the antero-lateral ganglion of the same side, but for the greater 

 part in that of the other side, describing a curve whose convexity 

 is directed backward, so as to give the impression that the ante- 

 rior ganglia are not related to one another, but only the posterior. 



But besides these medullated fibers, the commissure has another 



