“Prior to the year 1836, it was held by comparative anatomists that the brain in mammalia differed from that in all other vertebrate animals by the presence of the large mass of transverse white fibres called ‘corpus callosum’ by the anthropotomist; which fibres, overarching the ventricles and diverging as they penetrate the substance of either hemisphere of the cerebrum, bring every convolution of the one into communication with those of the other hemisphere, whence the other name of this part—the ‘great commissure.’ In that year I discovered that the brain of the kangaroo, the wombat, and some other marsupial quadrupeds, wanted the ‘great commissure’; and that the cerebral hemispheres were connected together, as in birds, only by the ‘fornix’ and ‘anterior commissure.’ Soon afterward I had the opportunity of determining that the same deficiency of structure prevailed in the _Ornithorhynchus_ (duckbill) and _Echidna_ (spiny ant-eater).” Owen’s conclusions with regard to the absence of the great connecting band of fibres between the hemispheres of the marsupial brain were first published in the _Philosophical Transactions_ for 1837; those, with regard to the same lack in the monotremes, being added in Todd’s _Cyclopædia of Anatomy and Physiology_, Article “Monotremata.” In the latter article it was also stated that the brain of the echidna was further distinguished from that of other mammals by the circumstance that whereas in the latter the portion of the brain known as the optic lobes consists of four lobes (_corpora quadrigemina_), in the echidna and duckbill there are only a pair of such lobes (_corpora bigemina_.) In consequence of this supposed lack of the corpus callosum in their brains, Owen separated the marsupials and monotremes from other mammals in a primary group by themselves, under the title of Lyencephala. Flower’s attack on these conclusions was commenced by a paper which appeared in the Zoological Society’s _Proceedings_ for 26th January 1864, entitled “On the Optic Lobes of the Brain of the Echidna,” in which it was conclusively demonstrated that these structures resembled those of the higher mammals in being four-lobed. More important still was his memoir “On the Commissures of the Cerebral Hemispheres of the Marsupialia and Monotremata, as compared with those of the Placental Mammals,” which was published in the _Philosophical Transactions_ of the Royal Society for 1865. In this was shown, it was thought, the existence in both monotremes and marsupials of a distinct, although very small, corpus callosum connecting the two hemispheres of the brain; the anterior commissure, which in the higher mammals is the smaller connecting band, being in this instance much the larger.