182 PROF. W. H, FLOWER ON THE ARRANGEMENT [Apr. 17, 



reckoned as orders. As the extinct Zeuglodon, as far as its charac- 

 ters 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 type\ 



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 Ungu- 

 lates, both perfectly natural and distinctly circumscribed groups. 

 The breakiug-up of the latter into four equivalent sections, the 

 Pecora, Tylopoda, TraguUna, 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 much is to be said in favour of this view. 

 The discovery, however, of a vast number of extinct species of Ungu- 

 lates which cannot be brought under the definition of either Peris- 

 sodactyle 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 necessai-y either to intro- 

 duce 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 

 the 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 Mr. Dob- 

 son's article in the ' Encyclopaedia.' They appear to resemble each 

 other in presenting a lower type of placentation 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 the chorion and conveys bloodvessels to it ; but the few ob- 

 servations 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 palseon- 



^ On the question of the origin and affinities of the Cetacea, see a lecture 

 delivered at the Eoyal Institution of Great Britain, May 25th, 1883, and pub- 

 lished in 'Nature,' June llSth and July 5th, 1883. 



