48 youriwl of Comparative Neurology and Psychology. 



Ahlborn, moreover, found the left fasciculus of Meynert to be 

 smaller than the right one. 



The fibers of this tract end in the basis of the mid-brain, in the 

 region where the oculomotorius roots leave the brain, in the corpus 

 inter pedujTculare. The decussation, which is a complete one there, 

 takes place in Galeus (Fig. Ixi, Plate IV) in three separate little 

 bundles which do not decussate at the same point, but one after 

 the other almost in the same level. After the decussation the 

 fibers gradually disappear in the basal gray substance of the mid- 

 brain described as the interpeduncular nucleus. I could not 

 determine that any fibers went farther back, as has been stated 

 to be the case in the cyclostomes and ganoids by some investi- 

 gators. 



Before leaving the epithalamus I have still to mention a decus- 

 sation which as far as I know, has not been separately described 

 in selachians by any one of my predecessors, but which has prob- 

 ably been considered by all as a part of the commissura posterior, 

 with which its fibers afterward are mingled. It consists of thin 

 but medullated fibers represented in Fig. liii, which do not decus- 

 sate like the com. habenularis before the epiphysis, but behind it 

 and before the folding of the dorsal wall up into the tectum. The 

 dorsal thalamus roof forms a somewhat deeper fold so that this 

 group of fibers is just as distinctly separated from the com. 

 posterior as the root of the epiphysis separates it from the com. 

 habenularis, which besides is situated at a higher level. Its 

 fibers, running downward, join the most anterior fibers of the 

 com. posterior and seem to end along with these. I cannot say 

 whether there may exist relations with the nucleus praetectalis or 

 whether this is the same bilateral connection which Holt men- 

 tions in the same region in the teleosts. Johnston, who saw a 

 similar commissure in Acipenser, describes it as the epiphysial 

 decussation, and states that these fibers partly originate from, 

 and partly end between, the epithelial cells of the epiphysial sac 

 and can be traced downward within and in front of the optic 

 bundles into the region of the nucleus anterior, as seems to be the 

 case here. 



Now, passing over from the epithalamus to the thalamus, in 

 Galeus I must mention a group of fibers (Fig. Ivi) which originate 

 laterally of the most anterior part of the thalamus immediately 

 under the insertion of the tectum, medially to the corpus genicu- 



