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Show 182 PROF. w. H. FLOWER ON THE ARRANGEMENT [Apr. 17, reckoned as orders. As the extinct Zeuglodon, as far as its characters are known, does not fall into either of these groups, but is in some respects an annectent form, I have placed it provisionally, at least, in a third group by itself, named Archceoceti. There is nothing known at present to connect the Cetacea with any other order of Mammals ; but it is quite as likely that they are offsets of a primitive Ungulate as of a Carnivorous type1. The remaining Eutherian Mammals are clearly united by the characters of their teeth, being all heterodont and diphyodont, with their dental system traceable to a common formula. Although older views of the relationship of Ungulate Mammals expressed by the terms Pachydermata, Ruminantia, and so forth, still linger in some corners of zoological literature, no single point in zoological classification can be considered so firmly established as the distinction between the Perissodactyle and Artiodactyle Ungulates, both perfectly natural and distinctly circumscribed groups. The breaking-up of the latter into four equivalent sections, the Pecora, Tylopoda, Tragulina, and Suina, is equally in accordance with all known facts. Less certain, however, is the association of the Proboscidea and the Hyracoidea with the true Ungulates. By many they are each, although containing so very few existing species, made into distinct orders; and m u c h is to be said in favour of this view. The discovery, however, of a vast number of extinct species of Ungulates which cannot be brought under the definition of either Perissodactyle or Artiodactyle, and yet are evidently allied to both, and which to a certain extent bridge over the interval between them and the isolated groups just mentioned, make it necessary either to introduce a number of new and ill-defined ordinal divisions, or to widen the scope of the original order so as to embrace them all, considering the Elephants and the Hyraces as representing suborders equivalent to tbe great Perissodactyle and Artiodactyle groups. It is the latter alternative that I have adopted. In the association of the three orders Insectivora, Cheiroptera, and Rodentia, and in their subdivisions, I have followed M r . Dob-son's article in the ' Encyclopaedia.' They appear to resemble each other in presenting a lower type of placentatiou to that of the other Eutherians, shown in the important part played by the umbilical vesicle, which becomes adherent to a considerable part of the inner surface of tbe chorion and conveys bloodvessels to it; but the few observations hitherto made upon this subject require to be confirmed and extended before it will be safe to attach much weight to them. This and other cranial and cerebral characters indicate that they occupy an inferior grade of development in the Mammalian series; but there are difficulties in interposing them in any other position than that assigned to them here, which must not be supposed to imply any superiority over the groups placed below them, but rather that they occupy a central position, connected, as paleeon- 1 O n the question of the origin and affinities of the Cetacea, see a lecture delivered at the Royal Institution of Great Britain, M a y 25th, 1883, and published in ' Nature,' June 28th and July 5th, 1883. |
