386 J. B. JOHNSTON 



campi. It is continuous rostrally with the undoubted primor- 

 dium hippocampi of figure 14 and is separated from the medial 

 olfactory ridge (area parolfactoria) by a continuous ventricular 

 sulcus. This sulcus has been overlooked by previous workers 

 because they have depended upon the study of sections. After 

 it has been seen in the dissected brain it can be traced in sec- 

 tions (figs. 11 to 17), but owing to its curve it comes to lie at one 

 point so nearly in the plane of transverse sections (fig. 15) as 

 to be very inconspicuous. 



The parolfactor}^ ridge above described has been figured by 

 Unger ('06) in sections of the brain of Gecko under the name 

 of nucleus septi, although he describes and figures it as a part 

 of the area parolfactoria. Kappers and Theunissen ('08, fig. 

 19) call this the nucleus accumbens septi and name the sulcus 

 which bounds it the fovea septo-striatica, considering the nucleus 

 as a part of the striatum. Herrick ('10) cites Kappers and Theu- 

 nissen but identifies the nucleus in Lacerta (fig. 43) with his 

 nucleus lateralis septi which is in reality part of the primordium 

 hippocampi. That is, he has overlooked the sulcus limitans 

 hippocampi of the above description and has combined the area 

 parolfactoria and the greater part of the primordium hippocampi 

 under the name nucleus lateralis septi. 



Upon the medial surface of the hemisphere of a turtle's brain, 

 the outlines of the area under consideration can be made out 

 almost equally well (fig. 20). The rostral portion of the lower 

 half of the wall is occupied by an almost circular area which is 

 continuous with the tuberculum olfactorium in the ventral wall. 

 This clearly corresponds to the medial olfactory nucleus or area 

 parolfactoria of the selachian brain. Dorsal to it are two sulci 

 (cf. figs. 13, 14, 15 and 20). The lower one continues upon the 

 ventral surface as the caudal boundary of the tuberculum olfac- 

 torium. The more dorsal sulcus extends from the olfactory sul- 

 cus back over the interventricular foramen. These two sulci 

 correspond to the two above described in the ventricular surface 

 of this wall. The more dorsal sulcus is the one called by Elliot 

 Smith the sulcus limitans hippocampi because it forms the ven- 

 tral boundary of undoubted hippocampal formation. The more 



