PROSENCEPHALON OF MAMMALS. 103 



back. 1 Like the water-shrews, the Ornithorliynchus has a smooth 

 cerebrum; the JEchichia, like the Great Ant-eaters and the 

 Sloths, has a convoluted one. Besides the long and deep ( hip- 

 pocampal fold,' the fore part of the mesial surface shows a 

 beginning of the supercallosal one ; behind which it is also 

 notched vertically by the mesial ends of the upper transverse 

 folds, 2 fig. 71. Of these, three nearly parallel ones extend 

 across the broad posterior part of the upper surface of each 

 hemisphere, their outer ends inclined forward ; anterior to them is 

 a larger convolution bent upon itself so as to form the inner 

 boundary of the anterior half of the upper surface. In the angle 

 of the above are two oblique folds inclining ( mesiad ' toward the 

 contracted fore part of the hemisphere. The base of the brain, 

 fig. 52, shows a few short foldings of the surface of the great 

 natiform protuberances, b'. The principal folds sink about a 

 line's depth into the substance of the cerebrum. The rhineiice- 

 phalon is enormous, ib. R. Some of the fibres of the great 

 anterior commissure bend forward, and are continued into each 

 of its crura. The outer part of the crus, ib. i , continued from 

 that of the prosencephalon, emerges from the fore margin of the 

 natiform protuberance, from which it has a reinforcement of fibres ; 

 the inner division, tumid with added grey neurine, ib. i b, is also 

 very broad. The prosencephalic cavity or ' ventricle ' is con- 

 tinued into the rhinencephalon, and is exposed in fig. 52, by re- 

 moval of the thin floor which rests upon the large 6 cribriform 

 plate.' The ( pineal ' and pituitary (ib. p) appendages of the 

 prosencephalon offer no monotrematous characters. 



There is not that difference of size between the Ornitho- 

 rliynchus and Echidna which would lead us to connect therewith 

 the convolution of the hemispheres in the latter animal ; what 

 is known of their habits suggests no superiority of psychical 

 power and resource in the land- over the water-monotrematous 

 Insectivore. Increased extent of the walls of the hemisphere in no 



1 My observations on this state of the ' corpora quadrigemina ' in Monotremes 

 accord with those of Laurent and Eydoux on the Echidna, and of Meckel on the 

 Ornithorhynchus. ' En comparant les tubercules quadrijumeaux de 1'Echidne a ceux 

 de 1'Ornithorhynque, nous avons facilement constate ce que 1'a deja ete par Meckel 

 pour ce dernier, c'est-a-dire qu'on ne peut pas distinguer les tubercules posterieurs 

 des anterieurs, et que ce que Meckel a remarque chez 1'Ornithorhynque et exprim6 

 en ces termes : " Eminentia quadrigemina magna, posterior tamen vere percipienda, ut 

 fere bigemina esset," est encore plus prononce dans les tubercules du cerveau de 

 1'Echidne, qui sont reellement bijumeaux simplement.' LVII". p. 164. 



- Well given in LVII". pi. ix. fig. 4 : omitted ill the diagram of a similar section 

 in XLIII". pi. xxxvii. fig. 7. 



