ANATOMY OF A CYCLOSTOME BRAIN 651 



mordium hippocampi is telencephalic, then the Hue of attach- 

 ment of the membranous roof must be regarded as taenia fornicis 

 (fig. 6, t.f.) in spite of its pecuhar form relations. We have 

 termed the fiber tract which borders this taenia the fimbria (figs. 

 3, 6, fim.), though it is very incompletely homologous with the 

 mammalian fimbria. Like the latter, it carries fibers from the 

 secondary oKactory area to the hippocampus, and also fibers from 

 the hippocampus to the habenula (tractus cortico-habenularis 

 medialis). But here, as in urodele Amphibia, this last connec- 

 tion is made much more directly than in mammals. 



The arrangement of the neurones of the primordium hippo- 

 campi is very characteristic, as Johnston has shown; the deter- 

 mination of the limits of this structure is therefore easily ac- 

 complished. Our preparations show the characteristic lamina 

 of hippocampal cells extending forward from the prehabenular 

 recess nearly tp the dorsal commissure. The ventral limit of this 

 lamina is well defined and under the rostral half of the primor- 

 dium this limit is marked by a sharp sulcus, the sulcus suhhip- 

 pocampalis (figs. 3, 6, s.shp.). In the caudal part of the primor- 

 dium its characteristic cells extend farther ventralward and the 

 limiting sulcus is not developed. The position of this ventral 

 border is indicated in figure 3 by a row of crosses. The posterior 

 border of the primordium hippocampi is marked by a deep groove, 

 the sulcus thalamicus 1. 



The lobus subhippocampalis. This is a well defined eminence 

 lying below the primordium hippocampi (figs 3, 6, l.shp.). It 

 is bounded above by the sulcus subhippocampalis (or by the 

 corresponding zona hmitans where this sulcus fails), below by 

 the sulcus medius, in front by the interventricular foramen, 

 and behind by the sulcus thalamicus 1. Its internal structure 

 is very different from that of any of the surrounding parts, so 

 that its limits could be easily defined even if there were no dis- 

 tinct ventricular sulci to mark them. 



Behind the subhippocampal lobe is the eminentia thalami, 

 whose cells are arranged in several parallel plates bordering the 

 ventricular surface, with scattered cells farther peripherally (fig. 

 7). Passing forward from this region, as soon as the sulcus 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 6 



