173 
THe LATERAL ASPECT. 
The cranial surface of the cerebrum (fig. 2) appears to be quite 
smooth, if one excepts a shallow furrow, which, starting in front 
in the sulcus between the hemisphere and bulb, arches backwards 
and downwards to reach the ventro-lateral border of the hemi- 
sphere at about its mid-point. This groove contains the external 
olfactory radiation of Edinger—the external olfactory root of most 
writers (fig. 1 ¢.0.7.). This radiation consists of a somewhat sickle- 
shaped band, of a pure white color, and in Wotoryctes is relatively 
very broad in front, but it rapidly tapers as it is traced back- 
wards. An analagous arrangement of this tract is found in all 
Marsupials, and most lowly Eutheria, but in the Monotremes it 
does not form a compact strand, as in the Marsupials, but a 
diffuse fibre system scattered uniformly over the pyriforin lobe. 
The depression in which it lies in Marsupials is the fisswra radicis 
of Herrick (17) or endorhinal fissure of Turner (42). In the 
case of Notoryctes and Perameles, where the radiation is broad, 
the fissura endorhinalis should properly be placed at the ventral 
or concave border of the strand, since the latter really rests upon 
the pyriform lobe, of which the endorhinal fissure is considered 
the ventral limit. Apart from this slight depression, corres- 
ponding to the fibre tract, the lateral surface of the hemisphere 
appears perfectly smooth and free from fissures. There does not 
appear to be any trace of a rhinal (ectorhinal) fissure. It is just 
possible that in a perfectly fresh specimen such a fissure may be 
discernible, but, arguing from comparative data, one should ex- 
pect it to be very indistinct and shallow, if present at all. The 
rhinal fissure forms the dorsal boundary of the pyriform lobe, 
and therefore is reckoned as a “fundamental fissure” (Turner), 
since it separates the pyriform from the “palliwm” (in the re- 
stricted sense used by Turner), which lies above it. The position 
of the fissure upon the brain surface will depend therefore upon 
the relative sizes of pallium and pyriform. Thus in many 
Marsupials, such as Dasyurus, Phalangista, Petaurus and Phascol- 
arctos, and in many Eutheria, such as the rabbit, it consists of 
a well-marked fissure on the lateral aspect, near the base of the 
brain. As the relative size of pallium to pyriform increases, as 
in Hypsiprymnus, Letrogale, Echidna, Macropus, Ornithorhynchus, 
and most placental Mammals, the fissure comes to be placed 
nearer the base, or even to lie entirely upon the basal or mesial 
aspects of the cerebrum. As, however, the proportion of pallium 
to pyriform decreases the position of the fissure on the lateral 
wall rises ; in other words, the pyriform lobe takes an increasing 
share in the formation of the lateral part of the hemisphere. 
Thus in Dasypus (Turner, 41), Zalpa (Ganser, 13), Didelphis 
(Herrick, 17) it is greater than in Dasyurus. In Hrinaceus the 
