350 J. B. JOHNSTON 
the dorsal border of the brain near the nucleus habenulae. This 
is more clear from my figure 9 which is drawn from a section 
between the two drawn in Herrick’s figures 80 and 81. In this 
section the stria medullaris is seen in the eminentia thalami. In 
Herrick’s figure 81 the stria medullaris has bent laterad into the 
outer portion of the nucleus habenulae and the tractus habenulo- 
peduncularis is coming down near the ventricle. Having thus 
identified the eminentia thalami, it is evident that the groove in 
Ichthyomyzon which corresponds to the suleus medius of amphi- 
bians is the groove so lettered in figure 6. 
Professor Herrick has completely overloooked the greater part 
of the suleus limitans hippocampi in Ichthyomyzon, probably 
because the greater part of its course lies more or less parallel 
with the plane of the transverse sections. For the same reason 
he failed to recognize that his s.m. and s.v. were the two ends of 
one and the same crescentic groove. The model clearly shows 
the true relations in both cases, and readily explains how natural 
Professor Herrick’s interpretation was in the absence of models. 
In dealing with the relations of the telencephalon and dience- 
phalon in the dorsal region, Professor Herrick (’10, pp. 473-4) 
says, 
On account of the very small degree of evagination of the cerebral hem- 
isphere in cyclostomes the di-telencephalic fissure is shallow and the pars 
dorsalis thalami passes over without interruption into the lateral wall 
(lobus olfactorius) of the hemisphere. Moreover this fissure does not 
extend upward to the mid-dorsal line and thus the dorso-median ridge 
is able to pass continuously from one segment to the other. In higher 
vertebrates this fissure extends dorsally up to the site of the velum trans- 
versum and it is so deep as to interrupt the continuity of both the ridge 
and all other massive tissue of the pars dorsalis thalami with their telen- 
cephalic representatives. 
It is necessary to define clearly what is meant by the di-telen- 
cephalic fissure. In the human and mammalian brain there 
is a great groove or fissure between the posterior part of the 
hemisphere and the thalamus, midbrain and cerebellum. Near 
the bottom of this is the chorioidal fissure of the hemisphere. 
Dorsally the fissures of the two sides join in the great longitudinal 
fissure. ‘Taken together these constitute in a true sense a margi- | 
