130 Mammalia. Nervous Syste7n. 



Prosencephalon. The Cerebral Hemispheres do not extend over the 

 Cerebellum in any of the species : their external surfaces are un-con- 

 voluted in most Phalangistidae, Peramelidae, Didelphidse (Opossum), 

 and small Dasyuridse 



xiv. Monofremata. JEpenceplialon. The grey matter does not form 

 a corpus dentatum in the olivary tracts. The lateral lobes of the 

 Cerebellum are very small, and, concomitantly, the Pons also, 

 especially in Ornithorhyncus, where it assumes the form of a narrow 

 band ; but the median lobe is large, and in Echidna forms the main 

 bulk of the Cerebellum. Mesencephalon, The posterior pair (testes) 

 of the corpora quadrigemina are not separated by a longitudinal 

 groove. Prosencephalon. In Ornithorhyncus the Cerebral Hemi- 

 spheres overlap the Mesencephalon and reach to the Cerebellum : with 

 the exception of the hippocampal fissure and the depression lodging 

 the Hhinencephalic crus, the surface is unbroken and smooth. In 

 Echidna the outer surface of the hemispheres is extended by convolu- 

 tions. The ventricles are continued into the Phinencephalon, which 

 in Echidna is enormous. 



3. Nerves, 

 {a.') Olfactory 



are ahsent 



in all the Cete save those with baleen, in which they are few and 

 small. 



exit from skull. 



In the Ornithorhyncus is the sole known instance of the Olfactory 

 nerves quitting the skull by a single foramen, i.e. one for each Rhinen- 

 cephalon. 



(5.) Optic. 



origin. 



The optic nerves seem to be derived more wholly from the thalami 

 in Man than in most lower Mammals. 



exit from the skull. 



In some Marsupials the optic nerve escapes by a cleft continuous 

 with the 'fissura lacera anterior.' 



size. 



The Optic nerves are smallest in the Moles ; largest in the Giraffe, 

 (c.) Trigeminal or Fifth. 



This nerve attains its largest relative size in Ornithorhyncus 



