1908.] AXTECHINOMYS AND OTHER MARSUPIALS. 597 



the inclusion of a particular type in that phylum. Dv. Mitchell's 

 sketch of the " archecentric " condition of the mammalian gut 

 agrees absolutely with the figures giv^en in any text-book of 

 Human Anatomy * of the early human gut, and any mammalian 

 gut. 



There is in fact no doubt whatever that the primitive 

 Mammalian gut was in all essentials a gut like that of the Reptilia, 

 i. e., a tube of no gi-eat length, and therefore with but few 

 convolutions suspended by a continvxous mesentery and with no 

 permanent folds of any part. I leave undecided whether a caecum 

 or cfeca are necessary adjuncts of this archetypal intestine, or 

 whether they or it should lie about halfway down the intestinal 

 tvibe. 



Greater or less length is clearly of no importance inasmuch as 

 that feature has been shown to vary in individuals (see above 

 p. 585). Viewing the matter from this point of vantage, we ought 

 to regard as most primitive in position any groups or group in 

 which the alimentaiy tract has retained this Reptilian character 

 throughout ; which in fact are so far not one geneiation removed 

 from the entire group of Lacertilia (including Hatteria), where no 

 other conditions ai-e, so far as anatomical investigation has gone, 

 to be found. 



So far as I can say from my own knowledge and from reliable 

 statements published upon the matter, the only groups in which 

 this primitive gut exists obviously are the Polyprotodontia 

 {excluding the American forms), the Xenarthra (excluding 

 Armadillos), the Proboscidea, the Odontoceti, and the Insectivora. 

 But with regard to the latter the case of Centetes described above 

 rather suggests a reversion. The Lemurs can hardly be added, 

 since Tarsius is the only form which shows this straight 

 mesentery unfolded anywhere ; and as that genus is so minute in 

 size the feature may be the result of degeneration. 



Why Dr. Mitchell should remove from such an assemblage t 

 the Insectivora, Proboscidea, and Odontoceti, and add to it the 

 Tubulidentata and Diprotodont Marsupials, is not altogether 

 ■easy to understand. His arrangement appears to me to be 

 so far purely capricious, and to be based upon no facts. More- 

 over, I would point out that very nearly all zoologists would agree 

 in regarding the groups which I have tlius placed in juxtaposition 

 as being ancient groups. 



Dr. Mitchell, however, appears to me to be perfectly right in 

 asserting that the Carnivora have not moved far from the common 

 centre ; though why this statement should be qualified by the 

 suggestion that the reduction of the hind gut is a specialisation 

 is not so apparent. As Dr. Lonnberg has well pointed out in the 

 case of certain Mai-supials % (and othei-s have pointed out in other 



* Cf.e.g. D. J. Cunningbam's Text-book. 



t According to him the " ancestral group " contains Marsupialia, Xenarthra, 

 Tnbulidentata, and Galeopithecids. All Marsupials, I presume, are included. 

 X p. Z. S. 1902, vol. i. p. 12. 



