175 
other words, it reaches its height of development early in ento- 
geny as in phylogeny, and has almost ceased growing, at a time 
when the continued active growth of the pallium becomes mani- 
fest by the production of gyri and sulci. Hence the pyriform 
always 1s and remains smooth.* Behind the external olfactory 
radiation the pyriform lobe is prolonged downwards to form a 
marked protuberance at the postero-inferior part of the hemi- 
sphere. This is the natiform eminence (fig. 1, n.e.) To this pos- 
terior part of the pyriform lobe, which is often called the lobus 
hippocampi, the term pyriform is often restricted (Turner 42), 
the anterior part of the lobe proper being considered part of the 
olfactory peduncle. An examination of the cerebrum of Platypus 
shows at a glance the inconsistency of this distinction. In 
Ornithorhynchus the pyriform lobe forms a uniform band extend- 
ing from the sulcus between bulb and hemisphere in front to the 
extremity of the latter behind, and throughout exhibits uniformly 
the characteristic features of the lobe, as described in the rabbit 
by Koelliker (15). The natiform eminence is largely the expres- 
sion of the bending, which takes place in the posterior pole of 
the hemisphere in Mammals. 
In Platypus, where the bending is slight, there is therefore no 
natiform eminence. In other Mammals, when the bending takes 
place, the pyriform, which occupies the concavity of the bend, 
tends to be crowded into a smaller space, and to overcome this 
tendency bulges posteriorly. This is all the more marked, since 
in all animals the pyriform (like the cerebrum itself) is larger and 
deeper posteriorly, in conformity with the shape of the skull. 
Another evidence of this bending is seen in the vallecula Sylvii, 
which Turner considers as the anterior limit of the pyriform lobe. 
It is probably merely the result of the bending. 
The term hippocampal, which is often applied to this region, is 
very unfortunate, and the cause of considerable confusion. The 
close relation of the pyriform to the hippocampus in Eutheria is 
largely fortuitous and not essential. In Marsupials in the greater 
part of its extent the subiculum cornu Ammonis does not form 
part of the gyrus hippocampi or of the pyriform lobe, this relation 
only obtaining at the posterior extremity (descending horn) of 
the hemisphere. In placental Mammals this is the only part of 
the hippocampus which survives the revolution in cerebral struc- 
ture, which the appearance of a corpus callosum creates ; and 
hence there is a close topographical relation between the whole 
of the hippocampus and the posterior part of the pyriform. But 
the pyriform lobe, lobus hippocampi, and gyrus hippocampi have 
*In certain animals, such as Hchidna and the pig, the surface of the 
pyriform lobe may become wrinkled. 
