The last of these can be dismissed, in consequence of the recent discovery of an allantoic placenta in _Perameles_. The first character is apparently a valid distinction between the Marsupials {117} 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.[65] 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 instance he observed that no Marsupial had more than a single successional tooth. It seems at the present moment to be fairly clear that Marsupials have a milk dentition like other Eutherians, but that only one of these teeth, the fourth premolar, comes to functional maturity. That it is really one of a complete milk series is evidenced by the fact that this tooth is differentiated contemporaneously with another series formerly held to belong to the so-called prelacteal dentition.[66] There still remains, of course, the actual fact that 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 in many other mammals admittedly Eutherian. [Illustration] FIG. 57.--Brain of _Echidna aculeata_; sagittal section. _ant.com_, Anterior commissure; _cbl_, cerebellum; _c.mam_, corpus mammillare; _col.forn_, column of the fornix; _c.qu_, corpora quadrigemina; _gang.hab_, ganglion habenulare; _hip.com_, hippocampal commissure; _med_, medulla oblongata; _mid.com_, middle commissure; _olf_, olfactory lobe; _opt_, optic chiasma; _tub.olf_, tuberculum olfactorium; _vent. 3_, third ventricle. (From Parker and Haswell's _Zoology_.) Huxley regarded the peculiarities in the reproductive organs {118} of the Marsupials as "singularly specialised characters," in no way intermediate in character. This view applies also to the pouch, which, as already stated, distinguishes the adults of that group. But the impossibility of using this last character as one of any importance has been shown by the discovery of rudiments of it in embryos of undoubtedly Eutherian mammals (see p. 18). [Illustration]