352 J. B. JOHNSTON 
In cyclostomes the di-telencephalic fissure is as clearly present 
as in other vertebrates. It is marked by the eminentia thalami 
to which the velum transversum is attached. In the ammocoetes 
of Petromyzon dorsatus, at least, the di-telencephalic boundary is 
further marked by an external groove (fig. 4). Professor Herrick 
is in error in his speculations regarding the continuity of the dorsal 
part of the thalamus and the telencephalon in cyclostomes (pp. 
474 and 477-8). The di-telencephalic fissure is slightly masked 
in eyclostomes because of the crowding back of the telencephalon 
against the diencephalon but the sulcus medius comes to the dorsal 
border here precisely as in amphibians and if the dorsal column 
is interrupted by the di-telencephalic fissure in amphibians, it is 
interrupted in just the same way in cyclostomes. 
Herrick states (p. 472) that he has indicated in figure 73 by a 
dotted line (s.d.) ‘a somewhat arbitrary boundary’ between his 
dorso-median ridge (my primordium hippocampi) and his pars 
dorsalis thalami. This dotted line does not correspond to the 
suleus limitans hippocampi or to any thing that I can find in 
Lampetra, Petromyzon dorsatus, Entosphenus or Ichthyomyzon. 
The dotted line is lettered sulcus diencephalicus dorsalis while in 
the text (p. 470) the dorsal is spoken of as synonymous with the 
sub-habenular sulcus. In amphibians (Herrick ’10, 431 and fig. 
22) the subhabenular and dorsal sulci are distinct but are regarded 
as two parts of a sulcus which separates the epithalamus from the 
dorsal part of the thalamus. In amphibians the dorsal is the more 
caudal segment of the common sulcus. In the Ichthyomyzon 
diagram the positions are reversed. In amphibians neither of 
these sulci has even a remote relation with the interventricular 
foramen, and both lie wholly within the diencephalon. In the 
Ichthyomyzon diagram the line s.d. is connected with the foramen 
and lies wholly within the telencephalon. This arbitrary line in 
Ichthyomyzon has therefore no relation to the suleus dienceph- 
alicus dorsalis of amphibians. 
Herrick regards the groove which extends rostrad from the 
foramen as a continuation of the suleus diencephalicus dorsalis. 
In this position there are two grooves in Ichthyomyzon and in 
Lampetra. One extends forward from the dorsal angle of the 
