392 J. B. JOHNSTON 



'10, fig. 5) in the brain of Ornithorhynchus beneath the fascia 

 dentata and seen in the same position in the brain of Didelphys 

 and the bat (figs. 22, 30, 43, 44). EUiot Smith ('03, p. 469) gives 

 this sulcus the name sulcus limitans hippocampi and points out 

 the error of writers on the reptilian brain who regard this as the 

 fissura arcuata. In higher mammals the growth of the corpus 

 callosum greatly disturbs this sulcus in the rostral half of the 

 hemisphere but the fimbrio-dentate sulcus is a well known land- 

 mark in that region of all mammalian brains where the primary 

 relations of hippocampal formation and fimbria are retained. It 

 persists throughout the length of the corpus callosum in the 

 bear (fig. 76) and the bat (figs. 43, 44). 



The sections of the turtle brain show that the body which 

 Meyer called septum pellucidum and which I have here called 

 primordium hippocampi is marked off from the area parolfac- 

 toria by a ventricular sulcus and a cell-free zone, is traversed 

 by the olfacto-cortical fibers and by the hippocampal commissure. 

 This body lies above the neuroporic recess and in all these re- 

 spects it agrees with the primordium hippocampi of selachians. 

 The only difference is that a part of the hippocampal primor- 

 dium of selachians has developed into hippocampal cortex in 

 reptiles. 



In Didelphys (fig. 29), as in the marsupials described by Elliot 

 Smith, the dorsal or anterior pallial commissure lies in the same 

 plane with the large anterior commissure and above the neuro- 

 poric recess. The mass of gray matter in which it is partly 

 imbedded projects into the ventricle and is separated from the 

 parolfactory nuclei rostrad by the ventricular sulcus limitans in 

 the manner described above. It is the residue of the primordium 

 hippocampi of selachians and is continuous with the fascia den- 

 tata and hippocampus above. The fibers of the anterior pallial 

 commissure bend dorsad through the primordium to emerge on 

 the ventricular surface of the hippocampus as the alveus. Above 

 the commissure is the recessus superior whose membranous walls 

 are attached to the dorsal surface of the commissure. This is 

 nothing more or less than a rostral pocket of the third ventricle 

 covered by tela chorioidea, formed by the commissure pushing 



