CHAP. VI EU THERIA—CORPUS CALLOSUM Val bi 7/ 
and their mammalian relatives higher in the series; but it is 
not a character that should have been made use of by Huxley, 
since he believed in the existence of a corresponding element in 
the Dog. As to the corpus callosum (Fig. 50, p. 77) being small, 
that seems to be not more than a slight difference of degree.! 
A number of other characters of secondary importance were added 
by Huxley to the weight of evidence which led him to form a 
group Metatheria for the Marsupials. Some of these, however, 
are now known to be not evidence in that direction. For in- 
stance he observed that no 
Marsupial had more than a ee aa 
single successional tooth. It 
seems at the present moment (tT 
to be fairly clear that Marsu- CN 
pials have a milk dentition i= Ca) 
like other Eutherians, but 
that only one of these teeth, 
the fourth premolar, comes 
to functional maturity. That emam |opl\ tubolf 
it is really one of a complete wentsa coljforn 
milk series is evidenced by Fie. 57.—Brain of Lchidna aculeata ; sagittal 
the fact that this tooth is section. ant.com, Anterior commissure ; 
cbl, cerebellum ;— ¢.mam, corpus mammil- 
differentiated contemporane- lare ; col.forn, column of the fornix ; c.qu, 
ously with another series corpora quadrigemina : gang. hab, ganglion 
habenulare ; hip.com, hippocampal com- 
formerly held to belong to missure ; med, medulla oblongata ; mid.com, 
Pay ee ar C middle commissure ; o/f, olfactory lobe ; 
the so-called prelacteal denti- opt, optic chiasma; tub.olf, tuberculum 
tion.” There still remains, of olfactorium ; vent. 3, third ventricle. (From 
course, the actual fact that Parker and Haswell’s Zoology.) 
the milk dentition is not for the most part functional, but its 
significance breaks down with these fresh discoveries. Of this 
Professor Osborn has remarked: “The discovery of the complete 
double series seems to have removed the last straw from the 
theory of the marsupial ancestry of the Placentals.” But Huxley 
did not lay much stress upon this matter of the teeth, since he 
observed that similar suppressions of the milk dentition were to 
be found ins many other mammals admittedly Eutherian. 
Huxley regarded the pecuharities in the reproductive organs 
* Moreover, the ‘‘ corpus callosum and the anterior commissure... in... Hrin- 
aceus and Dasypus are almost Monotreme-like.”’ 
2 See Wilson and Hill, Quart. J. Mier. Sci. xxxix. 1899, p. 427. 
