NERVOUS SYSTEM. VERTEBRATA. 149 



the surface. The roots of the hypoglossal nerve, however, 

 serve to indicate the lateral limit of each pyramid. There 

 is a diffuse crossing of the fibres of the pyramids, instead 

 of the compact decussations found in most mammals. 



The trapezoid body, which is such an obtrusive feature 

 of most mammalian brains, is not recognisable as such by 

 the naked eye in the Monotremes. 



The cerebellum is noteworthy for the small dimensions 

 of its lateral parts, and from the fact that its structural 

 plan cannot be certainly brought into harmony with that 

 which is common to all the Meta- and Eutheria. It, how- 

 ever, closely agrees with that of the Platypus. But there 

 is a large sessile floccular lobe, in marked contrast to the 

 pedunculated, encapsuled flocculus of the Platypus. 



In the left cerebral hemisphere the lateral ventricle has 

 been opened so as to expose the hippocampus. The latter 

 presents an appearance which is characteristic of the 

 Monotremes. For, although the hippocampus also extends 

 through the whole length of the lateral ventricle in the 

 Marsupials, as it does here, it is only in the Monotremes 

 that we find the largest and plumpest part of the hippo- 

 campal arc occupying the dorsal and cephalic position. 

 The caudo-ventral part of the hippocampal arc rapidly 

 tapers to a point in Tachyglossus, and even to a more 

 marked degree in Ornithorhynchus. 0. C. 1323*. 



Ziehen, Semon's Forschungsreise, Jena. Denkschr. Bd.vi. 

 1897, p. 4. 



D. 192. The left cerebral hemisphere of a Spiny Anteater ( Tachy- 

 glossus [Echidna] aculeatus) : also a coronal section of 

 another left hemisphere. 



A rough area in the centre of the mesial surface of the 

 hemisphere indicates the place from which the optic 

 thalamus was detached (fig. 41) . In front of this area the 

 elliptical sections of two commissural bands are to be seen 

 The lower or ventral commissure (COMM.V.) is much the 

 larger of the two and contains nerve-fibres corresponding to 

 the anterior commissure of other mammalian brains. But, 

 as the coronal section (fig. 42) [which passes through the 

 two commissures] clearly shows, the ventral commissure 



