No. I.] DESMOGNATHUS FUSCA. 25 1 



Gage (13) has noticed a similar projection in the Diemyctylns 

 and Amia and has called it the crista, appropriating a name 

 given by Wilder (56) for a similarly appearing object discovered 

 by him on the fornix just dorsal to the precommissure in the 

 adult cat and sheep and in the human embryo. In the Dcsmog- 

 natJuis, this part contains no fibers and appears to be nothing 

 more than an intrusion of the membrane, the pia, covered only 

 by a layer of endymal cells ; in one specimen there was a 

 distinct loop here. She also proposes the term callosal 

 eminences for what Herrick figures as the intra-ventricular 

 lobes. The terms crista and callosal eminence are employed 

 in this article for convenience and to avoid inflicting new 

 terms, and not because it is believed a strict homology can be 

 established. 



At least two conditions should be considered with regard to 

 the phylogeny of the callosum ; its size relative to that of the 

 cerebrum and its relation to the amount of the cerebral cortex. 

 In the birds it is weakly represented, although the cerebrum is 

 of considerable size. It is proportionately much less developed 

 in this class than in the Amphibia. Osborn attributes its small 

 size to the thinness of the mesal wall. It cannot be due to 

 the insufficiency of cortex, as this greatly exceeds that of the 

 Amphibia, and the birds, therefore, seem to be an exception 

 to these two important factors. 



A brief survey would seem to indicate that the precommis- 

 sure is the first in point of development and usefulness, and 

 that the callosal and hippocampal fibres practically arise simul- 

 taneously, and are of equal importance in these low forms. 

 Regarding these hippocampal fibres as the precursors of what 

 is commonly known as the fornix, it is worth noting that in 

 some mammalian brains, where the callosum has not developed, 

 there were only the precommissure and fornix found, and in 

 some perhaps not even the fornix; while in no case within the 

 writer's knowledge has a brain been described with the 

 callosum well developed and either or both of the other com- 

 missures absent. 



The rima, or transverse fissure, does not exist in the 

 Amphibia. In mammals, its general direction is caudo-lateral. 



