380 J. B. JOHNSTON 
Fig. 9 Ichthyomyzon concolor. A transverse section through the nucleus 
habenulae and eminentia thalami of the right side. The section cuts the eminen- 
tia thalami at its most prominent part, where it is bounded by the sulcus limitans 
hippocampi below and the sulcus subhabenularis above, as seen in figure 6. 
Fig. 10 Petromyzon dorsatus, adult. Transverse section through the inter- 
ventricular foramen. The primordium hippocampi is not so large as in Lampetra. 
At the lip of the foramen it is continuous with the wall of the hemisphere. 
Fig. 11 Same series as figure 10, section through the eminentia thalam1. 
Fig. 12 Entosphenus, ammoccetes of 35 mm. Transverse section through 
rostral part of foramen interventriculare. The section is quite oblique so that 
on the right side it passes considerably rostral to the foramen. The primordium 
hippocampi is small but occupies the same position as 1n other forms. The section 
falls at the Junction of the parapineal body and the left nucleus habenulae. 
Fig. 13 Same series as figure 12, section through the eminentia thalami and 
the habenular nuclei. Note the great size of the right nucleus habenulae. On 
the right side the primordium hippocampi is cut. 
