PHYLUM CHORDATA 



570 



is the commissure of the nee-pallium, but, as proved by their 

 mode of origin, to the fibres of the fornix, and they connect 

 together only the hippocampi, the fasciae dentatce, or specialised 

 lower borders of the hippocampi, and an area of the hemisphere 

 in front of the anterior commissure (pre-commissuml area} : they 

 thus constitute an olfactory or arcJiipallial commissure, since all 

 these parts belong to the olfactory region or archipallium of the 

 hemispheres. In the Mouotremes (Fig. 1203) the hippocampal 

 commissure is only very slightly bent downwards at its posterior 

 extremity. In most Marsupials (Fig. 1204) it bends sharply round 

 posteriorly and runs forward again, becoming thus folded into two 

 layers, dorsal and ventral, continuous with one another at a 

 posterior bend or splenium, similar to the splenium of the corpus 

 callosum. The dorsal layer of the 

 hippocampal commissure becomes 

 almost completely replaced in 



cbl 



Fro. 1205. Brain of Ornithorhynchus 

 anatinus, dorsal view (natural size) ; ell. 

 cerebellum ; olf. olfactory bulbs. 



Fm. 1206. Brain of Echidna aculeata, 



dorsal view (natural 'size). 



the Eutheria by the fibres of the corpus callosum, and the ventral 

 part persists in the shape of the psalterium or lyra. 



In Ornithorhynchus (Fig. 1205) the hemispheres are smooth ; in 

 Echidna (Fig. 1206) they are tolerably richly convoluted. Both 

 genera, but more particularly Echidna, are characterised by the 

 enormous development of the parts of the hemispheres (archi- 

 pallium) connected with the olfactory sense. In the lower Mar- 

 supials there are no convolutions (Notoryctes, Koala, Phalangers), 

 while in the higher the convolutions are numerous, though the sulci 

 are not very deep (Macropus, Fig. 1207). Among the Eutheria 

 there is a great range in the grade of development of the brain, 

 from the Rodents and lower Insectivores to the higher Primates. 

 In the lower types of Mammalian brain the cerebral hemispheres 

 are relatively small, do not overlap the cerebellum, arid have 



