174 
fissure is higher than in Talpa. In Perameles nasuta it is higher 
on the cranial aspect than in Didelphis or Hrinaceus ; and the 
pyriform lobe takes a greater share in the formation of the lateral 
hemisphere wall than in any other Mammalian cerebrum with 
whose description I am acquainted. Now, the size of the pyri- 
form lobe varies directly with that of the olfactory bulb. In 
Notoryctes the proportion of cerebrum to olfactory is smaller even 
than in Perameles. The natural conclusion to draw is that in 
Notoryctes the proportion of pallium to pyriform is smaller than 
in Perameles, 2.e., that the situation of the rhinal fissure (if such 
a fissure were present) would be still higher up on the lateral 
aspect than it is in the Bandicoot’s cerebrum. This opinion is 
borne out by histological examination. 
It is to be noted that the constancy, depth, and distinctness of 
this fissure are apt to become less marked in animals with a 
small pallium. This has been pointed out in a number of 
Rodentia and Cheiroptera by Turner (42), in Dasypus gymnurus 
by Hyrtl (22), and is also to be noted in Perameles nasuta. In 
animals with a slightly higher pallial development, however, it 
becomes deeper and better marked, and is more constant. The 
size of the pyriform lobe is fixed by that of the olfactory bulb. 
The size of the pallium, however, is a very variable quantity, and 
does not present any relation to the development of any sense 
organ. It is a higher “level” of nerve-matter built upon the 
lower, rhinencephalic part, but independent of the latter as far 
as its proportions are concerned. When in the lowly Mammal a 
“ nallium” first appears it probably, as in Votoryctes, forms merely 
a small dorsal cap lying on the pyriform lobe, and not separated 
from it by any fissure. The size of the pyriform is already deter- 
mined, and therefore fixed, but the pallium continues to increase 
to a variable extent in different Mammals. As it increases 
(Perameles, Didelphis) the pallium tends to bulge over the fixed 
pyriform, producing a rhinal fissure. As this tendency becomes 
greater (Phascolarctos, Dasyuwrus, Petaurus, Phalangista) the 
fissure becomes deeper and much more definite, so that in all the 
higher forms it remains as a deep and well-marked sulcus. With 
its increased growth the pallium overlaps the pyriform, which 
thus comes to lie upon the basal or median aspect of the hemi- 
sphere, as in Ornithorhynchus, with its small pyriform and large 
pallium (vide 36, fig. 1). The appearance of the rhinal fissure 
and the constancy of the pyriform lobe can thus be explained on 
developmental and comparative grounds, without resorting to 
groundless and far-fetched hypotheses, such as those of Hill (19). 
The pyriform lobe is practically smooth in all Mammals, because 
its size is determined by the relative size of the olfactory bulb, 
and its form fixed before the pallium becomes convoluted. In 
