418 J- B. JOHNSTON 



The term primordium hippocampi was first used by Elliot 

 Smith ('03) to designate that portion of the reptilian and am- 

 phibian hemisphere which corresponds to the mammalian hippo- 

 campal formation. When the writer realized that the body in 

 the brain of fishes which he had called 'epistriatum' was in real- 

 ity the forerunner of the hippocampus, he adopted the name pri- 

 mordium hippocampi. At that time the writer supposed that 

 his primordium hippocampi in fishes was approximately equiva- 

 lent to Elliot Smith's primordium hippocampi in reptiles. It now 

 appears that the primordium hippocampi of the writer includes 

 the body to which Elliot Smith gave the same name plus the 

 equivalent of the septum. 



The most important matter is that the exact meaning of terms 

 be understood. Upon the lesser question as to what terms are 

 most appropriate a few words may be said. Elliot Smith's pri- 

 mordium hippocampi is the equivalent of the hippocampus and 

 fascia dentata. In the reptilian brain it is bounded by a sulcus 

 which he called sulcus limitans hippocampi. This name is lit- 

 erally appropriate and it is now clear that the sulcus in selachians 

 to which the writer applied the same name is an entirely different 

 sulcus. The sulcus limitans hippocampi of the frog as recog- 

 nized by Herrick and the writer is the same as that in reptiles. 

 It is shown in this paper that this sulcus lies between the fascia 

 dentata and the fimbria. Since the term sulcus fimbrio-dentatus 

 is in common use and clearly understood in descriptions of the hu- 

 man brain, it can be used for this sulcus in reptiles and amphib- 

 ians and the term sulcus limitans becomes unnecessary. There is, 

 however, need for a term to designate the boundary between the 

 pallial and sub-pallial areas in the medial wall. This is what Elliot 

 Smith attempted to do by his term sulcus limitans. It is to this 

 sulcus which limits the pallium that the writer has applied the name 

 sulcus limitans hippocampi. This term is inappropriate because 

 of the extreme divergence in structure between the hippocampus 

 and septum in mammals, although they are indistinguishable in 

 selachians, and remain similar in function. What is needed is 

 some term to indicate that the septum belongs to the pallial 

 area. This suggests such terms as sulcus marginalis pallii or 



