THE TELENCEPHALON IN CYCLOSTOMES 345 
deep sulci where they join in the caudal and lateral angle of the 
dorsal sac is a very deep and narrow cleft which may be called 
the recessus praehabenularis (figs. 14, 18). This deep recess has 
been formed probably by the crowding together of the brain due 
to pressure from in front. Below the habenular body two nearly 
vertical ridges are seen (figs. 5, 6) in the side wall of the thalamus. 
The more caudal one is occupied by the tractus habenulo-pedun- 
cularis (figs. 21, 22). It is much more prominent on the right, 
owing to the greater size of the nucleus habenulae and the fiber 
tract on the right side. The more cephalic ridge is about equally 
developed on the two sides and contains the stria medullaris (figs. 
20, 21). This I shall call the eminentia thalami. It is bounded 
in front by the sulcus limitans hippocampi and extends up in the 
recessus praehabenularis as a narrow ridge (figs. 20, 26, 27). The 
groove separating the two ridges corresponds to the sulcus b of 
the selachian brain (Johnston 711 a) and to the sulcus diencephal- 
icus medius of Herrick (’10) in amphibians. 
The dorsal ridge in front of the habenular body is large in 
Lampetra and presents a slight but distinct eversion similar to 
that in the teleost brain (figs. 7, 30, 31). To this ridge the writer 
formerly (02 a) gave the name epistriatum. In anticipation of 
the results of the following pages, we may call it here the primor- 
dium hippocampi. It extends forward over the interventricular 
foramen where it becomes continuous with the roof of the lateral 
evagination. 
In order to determine whether this ridge belongs to the telen- 
cephalon or diencephalon we must discover the point of attach- 
ment of the velum transversum to the massive walls. This point 
is difficult to determine in most petromyzonts because the velum 
is very rudimentary and is recognizable only near the median line. 
This is true of the adults of Petromyzon dorsatus and Lampetra 
which the writer has examined, of Ichthyomyzon studied by Herrick 
and apparently of the forms studied by European authors. In 
the ammocoetes of Petromyzon dorsatus, however, the velum is 
better developed and appears as a fold which extends across the 
whole width of the tela so that the attachment to the massive 
walls can be determined. 
