1910.] "Meseutheria" and "Cceneutheria." 461 



donta) were later (1909, p. 33, footnote c) referred to the Mcseutheria. 

 Whether or not the Ta'niodoiits are structurally ancestral to the Xenarthra, 

 all the Edentate orders exhibit indications of d(;rivation from some Mesozoic 

 order (p. 341). 



If the foreo;()ing conclusions be correct the more general conclusions in 

 regard to the status of the conception designated by the terms Meseutheria 

 and Caeneutheria appear to be as follows: 



(1) The idea is valuable in so far as it centers attention upon the fol- 

 lowing inferences: (a) the existence of a great Mesozoic mammalian radia- 

 tion, (b) the extinction of the majority of the Mesozoic families, (c) the 

 retarded development of the brain especially in the Amblypoda and the 

 lowly character of the brain in all the Meseutheria; (d) the non-derivation 

 of the Lower Eocene families from the more specialized Basal Eocene 

 predecessors; (e) the inference that as a whole the Lower Eocene fauna of 

 Western North America represents a fresh immigration from the north; 

 while the Basal Eocene fauna in part moved southward, in part lingered on 

 in competition with the higher invading types, and in j)art gave rise to other 

 higher types. 



(2) On the other hand it would appear inaflvisable to apply the terms 

 Meseutheria and Caeneutheria in a systematic sense, for the following 

 reasons : 



(a) The names virtually stand for the inferred relations of two limited 

 faunas and do not represent systematic groups which can be properly defined. 

 Neither of the groups are held together by characters drawn from various 

 parts of the organism and are as originally stated by their author "very 

 heterogeneous." They are separated (incompletely) chiefly by their position 

 in time and by an assumed dift'erence "in the capacity for progressive evolu- 

 tion." 



(b) If understood in a systematic sense the two groups Meseutheria and 

 Caeneutheria appear to overlap each other at many points, both structurally 

 and phylogenetically. 



(c) The division would thus be largely "horizontal" in character, 

 analogous to Bonaparte's Ineducabilia and Educabilia, only not as well 

 defined, since the Ineducabilia included all the macrosmatic, small brained 

 modern groups. 



(d) Such a division would obscure the fact that the large brained forms 

 must have been derived from small brained forms, and that the ancestors 

 of the Insectivore-Creodont group were also the ancestors of many if not all 

 of the higher orders. 



