SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 385 



is hippocampal cortex. The small body between is undifferen- 

 tiated primordium hippocampi. The shght sulcus dorsal to it 

 is the sulcus limitans hippocampi of Elliot Smith ('03). Its 

 morphological significance will be discussed later. The deeper 

 sulcus below corresponds to the sulcus limitans hippocampi as 

 that term was used by the present writer ('11 a) for selachians. 

 A cell-free zona limitans extends through the medial wall oppo- 

 site to it. 



The question at once arises, how can the same ridge or thick- 

 ening in the medial wall be the medial olfactory nucleus at the 

 rostral end of the brain and the primordium hippocampi at the 

 foramen interventriculare, especially since a hippocampal pri- 

 mordium lies dorsal to this ridge at the rostral end of the brain? 

 With the exception of Meyer and of linger who distinguished 

 between the septum pellucidum and the olfactory nucleus C'vor- 

 deres mediales ganglion," nucleus septi), previous authors have 

 considered this whole region as belonging to the paraterminal 

 body. An examination of the entire brain at once shows the 

 error of this assumption. When the lateral ventricle of an adult 

 turtle's brain is opened by removing the lateral wall, the ven- 

 tricular surface of the medial wall shows the structures seen in 

 figure 19. Rostrally the ventricle is constricted at the level of 

 the olfactory peduncle by a transverse fissure dorsally and by 

 a large longitudinal ridge in the ventro-medial wall. This ridge 

 continues caudad in the lower part of the medial wall and is the 

 medial olfactory nucleus or area parolfactoria seen in figure 14. 

 Above this ridge is a groove in which under a hand lens there 

 are to be distinguished two ventricular sulci corresponding to 

 the two sulci on the medial surface above mentioned (fig. 14). 

 Of these sulci the dorsal one continues caudad in nearly a straight 

 line above the level of the interventricular foramen. The more 

 ventral sulcus, about midway between the olfactory peduncle 

 and the foramen, turns rather abruptly ventrad, forming a 

 rounded caudal boundary to the olfactory ridge, and then bends 

 caudad to reach the interventricular foramen (fig. 19). The body 

 lying between these two grooves is the ridge seen in figure 17 

 and hypothetically identified above as the primordium hippo- 



