304 | J. B. JOHNSTON 
The brain of Professor Reighard’s dwarf lamprey presents 
larger hemispheres with wider lateral ventricles than I have seen 
in any other petromyzont. In the form and size of the primor- 
dium hippocampi and in the disposition of the ventricular sulci 
it agrees well with Lampetra. 
The primordium hippocampi 
The description of the ventricular sulci has made clear the 
definite ventral boundary of this body on the ventricular side. 
Externally a very deep groove separates it from the caudal pole - 
of the hemisphere (fig. 7). At the interventricular foramen the 
primordium hippocampi bends through the roof of the foramen 
to become directly continuous with the roof of the hemisphere. 
Caudally, the sulcus limitans separates this body sharply from the 
eminentia thalami and nucleus habenulae internally, but on the 
external surface there is no visible boundary in adult Lampetra. 
In the ammocoetes of Petromyzon dorsatus the external surface 
shows a vertical groove which marks the caudal boundary of the 
telencephalon (fig. 4). 
The neurones of this body belong to a special type which is 
found nowhere else in the brain of Lampetra. The same type of 
neurone is characteristic of the primordium hippocampi of ganoids 
and amphibians. As far as the writer’s studies have gone, these 
neurones are as truly characteristic of the primordium hippocampi 
as are the Purkinje cells of the cerebellum, and are much more 
highly differentiated in cyclostomes than are the Purkinje cells. 
In a fomer paper (02 a, p. 40) these neurones as they appear in 
Golgi preparations were discribed as follows: 
The cells of the epistriatum are arranged in two to four rows adjoining 
the cavity. The larger end of the pyramidal cell body is next the cavity 
and a large dendrite which arises from the apex divides into two or more 
large branches which expand in the fiber layer. The dendrites bear 
numerous small spines which are knobbed at the end (fig. 26) in the 
manner characteristic of the epistriatum, inferior lobes, and tectum of 
Acipenser. These peculiar spines are found nowhere in the brain of 
Petromyzon except on the epistriatum cells. These cells are so closely 
similar to the epistriatum cells of Acipenser that it would be impossible 
to mistake their identity. 
