538 
motor area of the thalamus. The only objection which Kappers finds. 
to the hypothesis is that we cannot a priori expect to find a corpus 
callosum in animals so low as the selachians. I wish to point out 
that the leap from selachians to mammals is not so great as is usually 
considered. Neither ganoids, teleosts, amphibians, nor any living reptile 
stands directly in the line of mammalian ancestry. Mammals are 
connected with selachians through some ancestral forms common to 
mammals and reptiles. Many more studies of the various vertebrate 
classes must yet be made before any hypothesis regarding the homo- 
logy of centers or tracts in the brain of selachians with similar struc- 
tures in the mammalian brain can be summarily dismissed because of 
the distant relationship between selachians and mammals. 
The recognition by the writer of a posterior pallial commissure 
in fishes also calls for criticism from Kappers. He says: “Wir schreiten 
jetzt zu einer zweiten Angabe dieses Autors, der Homologisierung der 
marklosen Commissura superior (habenularis) der Haie mit der Com- 
missura pallii posterior”. He quotes from ErLior Smitrn’s “The 
Cerebral Commissures in the Vertebrata” (p. 484): “OSBORN ..... 
confused the commissura aberrans of reptiles with the commissura 
habenularum in the frog”, and says that I have now repeated OSBORN’S 
error. 
The commissura superior is not “marklos”, but contains an un- 
medullated bundle. I analyzed the stria medullaris into six compo- 
nents, five of which contain medultated fibers and contribute to the 
commissura superior. The sixth component is unmedullated and forms 
what I have termed the commissura pallii posterior. I have, there- 
fore, neither homologized nor confused the commissures mentioned. 
I have shown simply that a true commissure of the hippocampus, the 
commissura pallii posterior, constitutes one bundle in the commissure 
complex known as the commissura superior. Prof. C. Jupson HERRICK 
has given exactly the same interpretation of this bundle in amphibians 
(Herrick 1910, p. 427). If KAPPERs wished to cite ELLIOT SMITH’s 
position relative to this question he should have quoted the following 
from p. 494 of the same paper and the same paragraph from which 
he made one quotation: “From examination of the brain in a number 
of Amphibia and related forms, it seems certain that an analogous 
commissure is found there also, but that instead of pursuing an. in- 
dependent course it crosses the roof of the third ventricle alongside 
the fibres of the commissura superior of OsBorn. This phenomenon 
is readily understood when it is remembered that the thalamic’ region 
is very short in the Ichthyopsida, and the situations of the commissura 
