50 



upon the lateral aspect of the tuberculum olfactorium ; but posteriorly 

 it forms the ventro-mesial border of the hemisphere and in this region 

 its grey matter is continuous (in Ornithorhynchus) with that of 

 the corpus striatum — the bridge being apparently the homologue of 

 the nucleus amygdalae. 



In all Mammals the caudal pole of the hemisphere bends down- 

 wards to form a descending cornu. This bending has a marked in- 

 fluence upon the shape of the hippocampus and pyriform lobe, so that 

 in Platypus, where the bending is very slight, the appearance of the 

 unbent pyriform and hippocampus is very paradoxical to the student 

 of the Eutherian brain. In all other Mammals the bending is much 

 more extensive and a well developed descending cornu, which curves 

 downwards and forwards exists. As a consequence, the posterior pole 

 of the hippocampus curves downwards and forwards and its commissure 

 is correspondingly curved (Fig. 1 ps. d, ps. v). In Marsupials the 

 hippocampus forms a regulary arched structure, whose contour is indi- 

 cated externally by the hippocampal fissure {h. fiss.). Concentric 

 with this are the fascia dentata (f, d.) and the fimbria (fi.) in their 



whole extent. In the basal part 

 of the hemisphere the pyriform 

 lobe becomes bent up and crow- 

 ded into a smaller space. This 

 curving is felt all the more 

 ^l /'ss since the posterior part of the 

 pyriform is larger than the an- 

 terior. As a consequence the 

 caudal bent part projects as 

 a large f swelling — the "nati- 



G ES. 



Fig. 1. Mesial view of cerebrum of Perameles nasuta. ^ 2. 

 s lamina terminalis (septum lucidum) 5 /. d fascia dentata; a.c anterior commissure ; 

 ps bippocampal commis:>ure. 



form eminence" or "hippocampal lobule". To this swelling most writers 

 confine the term "pyriform lobe" including the rest of the lobe in the 

 olfactory peduncle (Sir W. Turner). The inappropriateness of such a 

 distinction is clearly shown in Ornithorhynchus where, on ac- 

 count of the slight degree of bending, the pyriform lobe (as defined 

 here) consists of a uniform band (with the characteristic histological 

 features [Koelliker]), which extends right forward to the olfactory 

 peduncle (in the restricted sense employed here). 



The tuberculum olfactorium (Koelliker) (Fig. 1 t. o.) is 



