342 J. B. JOHNSTON 
chiasma ridge the lamina terminalis extends from the preoptic 
recess to the recessus neuroporicus, rostral to the interventricular 
foramen. The position of the recessus neuroporicus was first 
clearly established by Sterzi (07). The lamina terminalis is 
thickened by the anterior commissure. Above the recessus neuro- 
poricus the roof of the ventricle is thickened by the so-called 
dorsal olfactory commissure or decussation. This thickened 
lamina should be called the lamina supraneuroporica (Burck- 
hardt 94 b, ’94 ¢, ’07, Johnston, ’11b). <A short distance caudal 
- to this lamina occurs a small fold in the membranous roof which 
was shown by Sterzi (’07) to be the velum transversum. Caudal 
to this a long dorsal sac extends to the superior commissure and 
habenular bodies. This dorsal sac is covered dorsally by the 
parapineal body and epiphysis, which depress the sac in various 
degrees. 
Upon the general morphology of the petromyzont telencephalon 
thus far there is general agreement among workers. There are, 
however, disputed questions regarding the relations of the telen- 
cephalon and diencephalon, and the location of the primordium 
of the cortical area of higher vertebrates. 
The writer has reviewed the preparations previously studied 
and has examined new preparations of the same and other species 
of petromyzonts. The writer wishes to express his sincere thanks 
to Professor Gage for a very generous supply of ammocoetes and 
adults of both Lampetra and Petromyzon dorsatus. Other speci- 
mens of Lampetra have been obtained from Professor Reighard’s 
laboratory both while the writer was located there and later 
through the kind assistance of Dr. L. J. Cole. The writer is — 
deeply indebted to Professor Charles Brookover for the loan of 
the series of sections of Ichthyomyzon which is described beyond. 
Thanks are also due to Professor Reighard for the loan of a series 
of sections of a new dwarf lamprey not yet described. Through 
the kindness of Mr. W. F. Allen of this laboratory I have had the 
opportunity to section and study several stages of the ammocoetes 
of the Pacifie coast lamprey, Entosphenus. 
