450 C. JUDSON HERRICK 



photographs (figs. 2 and 14 of Elliot Smith's paper) show clearly- 

 above this fissure B a denser mass of cells separated from the over- 

 lying pars dorso-medialis by a zona limitans. These cells mark 

 the most dorsal limit of the nucleus medianus septi or precom- 

 missural body, as I have defined it in this paper, and seem to be 

 exactly comparable with the larger mass of cells seen in larval 

 and adult frogs in the corresponding position. They may extend 

 caudad for a short distance above the interventricular foramen, 

 though the figures suggest that this is improbable. 



Elliot Smith explains briefly the morphology of the rudimentary 

 pallial formation of Lepidosiren and I shall return to the consider- 

 ation of his valuable suggestions in the final discussion (p. 490). 

 None of the available descriptions of the brain of Ceratodus are 

 well adapted for the comparisons here instituted. Bing and 

 Burckhardt ('05, fig. 15, p. 550) give a figure of the embryo which 

 shows that at this stage the cross section of the forebrain at the 

 level of the interventricular foramen is almost identical with that 

 of the just hatched Necturus (cf. Kingsbury, '95, pi. ix, fig. 8). 

 Protopterus, as figured by Burckhardt ('92) is much more sim 

 ilar to Lepidosiren than is Ceratodus. 



The fiber tracts of the Dipnoi are not sufficiently well known 

 to permit us to control all of the homologies here suggested; but 

 so far as known they support them. Burckhardt 's meager ac 

 count of the forebrain tracts of Protopterus shows that the com- 

 missura pallii anterior is present (termed corpus callosum), but 

 it is not clear whether it enters the lamina terminalis behind the 

 interventricular foramina as in Amphibia or in front of them as in 

 elasmobranchs. His figures of cross-sections show that the dien- 

 cephalon and telencephalon are divided longitudinally into parts 

 which correspond closely with those of Lepidosiren and Amphibia. 

 He discusses (p. 25) these longitudinal zones in comparison with 

 those of urodeles, but since he incorrectly homologized all of the 

 parts of the hemisphere (see Elliot Smith, '08), his analysis is 

 not fruitful. 



The cerebral hemispheres of the higher ganoids and toleosts 

 have developed in a different direction from that taken by the 

 Amphibia. In the olfactory bulb alone is the massive wall of 



