This investigation on the morphology of the Fore-brain or Prosencephalon of Elasmobranchii was suggested to me by Professor G. Elliot Smith, F.R.S., and I am indebted to him for introducing me to an interesting if somewhat difficult and perplexing subject. A glance at the literature on the morphology of the prosencephalon will readily convince one of two things—firstly, of the endless field for work in endeavouring to ascertain the true homologies of the neurone areas and fibre tracts in the brain throughout the Phylum Vertebrata; and secondly, of the somewhat unnecessary obstacles which have been introduced into the problem by the more recent additions to a terminology, already overburdened and encumbered, of new terms for equivalent parts. It would appear to be very advantageous if mutual agreement could be arrived at among workers on this subject regarding the use of descriptive terms, and, if such is at present impracticable, that each new worker should refrain as far as possible from applying a new term to a structure already previously described. In one case, namely, that of the tuberculum olfactorium, five different terms have apparently been applied to the same area.