SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSUKES 387 



verftral sulcus corresponds to the sulcus limitans hippocampi 

 described by the writer in selachians. The part of the medial 

 wall between the two sulci is an undifferentiated portion or ves- 

 tige of the primordium hippocampi of the selachian brain. 



The primordium hippocampi thus outlined contains the com- 

 missura hippocampi and together with the hippocampal cortex 

 recognized by previous workers, is the equivalent of the primor- 

 dium hippocampi of selachians. Compare figure 85. 



In Alligator mississippiensis the features described above are 

 repeated so exactly that it is unnecessary to present separate 

 drawings. There are differences in general form, and the area 

 parolfactoria is relatively smaller than in the turtle. 



The brains of various mammals, embryonic and adult, have 

 been examined with reference to the gross relations of the pri- 

 mordium hippocampi and the area parolfactoria. In the opossum 

 (Didelphys virginiana) these structures have essentially the same 

 form as in the turtle. The chief difference is that the area 

 parolfactoria is less prominent in the lateral ventricle and the 

 sulcus limitans hippocampi is broader and more shallow. The 

 medial wall as seen from the lateral ventricle is drawn in figure 

 21, which should be compared with figure 19. The medial sm-- 

 face of the same hemisphere is shown in figure 22. In this it 

 will be seen that the only important difference from the turtle 

 brain is that there are two longitudinal grooves above the fora- 

 men interventriculare, one above the fascia dentata, the other 

 below it. Compare Elliot Smith's figure of the brain of Orni- 

 thorhynchus ('98, fig. 2). The lower of these sulci separates the 

 fascia dentata from the fimbria and is therefore the fimbrio- 

 dentate sulcus. The upper one is the fissura hippocampi. The 

 area parolfactoria is prominent but relatively smaller than in 

 the turtle. The primordium hippocampi stretches forward from 

 the dorsal commissure and together with the fascia dentata ex- 

 tends into the medial wall of the olfactory peduncle. Compare 

 figures 25 to 28 and figure 86. 



These structures have been studied in a number of mammals. 

 Figures 23, 42, 50, 60, 67, 73 show that in the rat, rabbit, striped 

 gopher, bat, mole, and bear the relations are essentially the same 



