ANATOMY OF A CYCLOSTOME BRAIN 657 



was therefore placed at our sulcus medius (his sulcus limitans 

 hippocampi) . The larger specimen which forms the basis of the 

 present report, however, shows not only a much clearer internal 

 difference between these regions but throughout most of their 

 extent a well defined ventricular sulcus between them, the sul- 

 cus subhippocampalis. Our sulcus medius, therefore, is not the 

 limiting sulcus of the hippocampus, nor is the latter (the sulcus 

 subhippocampalis) to be compared with Elliot Smith's sulcus 

 limitans hippocampi of mammals, for the latter separates the 

 hippocampus from the medial olfactory area in the septum of an 

 evaginated hemisphere, structures which here are separated by 

 both the lobus subhippocampalis and the striatum (cf. Herrick 

 '10, p. 465). 



The evidence recently brought forward by Johnston ('12) ren- 

 ders it probable that in Herrick's discussion of the cyclostome 

 brain ('10, p. 473) the di-telencephalic boundary was placed too 

 far forward. It probably lies in our specimen in about the 

 plane of the sulcus thalamicus 1, thus defining both the primor- 

 dium hippocampi and the lobus subhippocampalis as telence- 

 phalic structures. 



The homologies of the lobus subhippocampalis in higher brains 

 can be definitely determined only after its functional connections 

 in cyclostomes are better known and the method by which the 

 process of evagination of the cerebral hemispheres was accom- 

 plished has been definitely determined. Even in the case of the 

 primordium hippocampi, comparisons with the hippocampus of 

 higher brains should be made with caution. That this is the 

 region from which the amphibian and the amnio te hippocampus 

 has been differentiated seems well established; but the relations 

 of this structure and the lobus subhippocampalis to each other 

 and to the mammalian hippocampus and pyriform lobe (among 

 other qustions) require further elucidation. 



The morphological problems presented by the diencephalon of 

 cyclostomes are more obscure than those of the telencephalon, 

 chiefly by reason of our very imperfect knowledge of the structure 

 of the thalamus in all lower vertebrates. That there is a consid- 

 erable regional functional differentiation within the thalamus of 



