422 J. B. JOHNSTON 



toria, Beccari '11). A discussion of the relations and terminology 

 of the region surrounding the tuberculum olfactorium must be 

 reserved for another time. 



The general results of the foregoing study should be summa- 

 rized lest the interest in various details should overshadow the 

 main significance of such a comparative study. The reptilian 

 and mammalian equivalents of the pallial and basal areas of the 

 selachian telencephalon and the boundary between them have 

 been accurately defined so far as the medial wall is concerned. 

 It has been shown that the septum pellucidum or its equivalent 

 is a derivative not of the basal olfactory area but of the pallial 

 area. It is the unchanged residuum of the selachian pallium 

 after the hippocampal formation is developed. The fusion of the 

 two septa (hippocampal primordia) in the lamina supraneuro- 

 porica forms the bed for the passage of the pallial commissures 

 between the hemispheres. It is shown that the two commis- 

 sures in the selachian pallium hold essentially the same relations 

 to all structures in the median region as are held by the hippo- 

 campal commissure and corpus callosum in mammals. It is 

 therefore strongly probable that the commissure of the somatic 

 area in the selachian telencephalon is the true forerunner of the 

 mammalian corpus callosum. The final determination whether 

 this is true or not will come with the further study of the history 

 of the somatic area and the clear demonstration of corpus cal- 

 losum fibers in reptiles and dipnoi. 



The writer wishes to correct here, with due apology to the 

 authors concerned, an unfortunate slip made in an earlier pub- 

 lication. In the paper on the selachian brain ('11a, p. 58, sec. 

 40) occurs a sentence which should read as follows: "The terms 

 neopallium (Elliot Smith) and archipallium (Edinger) are there- 

 fore not appropriate." I was acquainted with Professor Smith's 

 disclaimer of the term archipallium (Anat. Anz., Bd. 35, p. 429) 

 and it was my express intention in the sentence quoted to credit 

 the two terms to their proper authors. Through some inexpli- 

 cable error the names became interchanged and the mistake was 

 not noticed until recently. 



