THE AMERICAN NATURALIST. [Vol. XXXVI II. 



marsupials, a.s Dollo and Bensley have shown/ are probably 

 descended from an arboreal type which must have closely 

 resembled the arboreal ancestors of the placentals, but with 

 distinction>s in the number and succession of the teeth, the 

 ()ti,<,nn of which is not yet clear. In these, as in the placental 

 mainin.ils, ihc modem arboreal forms are the most primitive. 

 l>ut It ^hMlll(l 1)0 lemembered that we know comparatively little 

 ''i"»nt tlu. p>iKu)iUology of the marsupials earher than the Qua- 

 tuiiMi N, wlul^ \\c are able m most of the families of Ungulata, 

 Caini\,.ia. Rockntia and some other orders of placentals, to 

 ti>u. hatk tlK-n ancestry into the Middle or Lower Tertiary, 

 and lind thcni in every case converging toward the type char- 

 atteii/ed ai)ove, and quite closely converging as we come into 

 the Basal Tertiary. We are thus enabled to place the earliest 

 divergence from this central type in the Middle or Upper Cre- 



It may almost be taken tor granted that if the characters of 

 this central t>pe were in all respects as stated above, it must 

 have been of arboreal habitat. All its adaptations would be 



suitable 



xle of life, and some would be i 



unsuitable for any other. The various modern groups (monkeys, 

 S(iuinels, ai boreal insectivores, opossums) which have retained 

 thi> habitat aic the least altered in structure, while the amount 

 " ^--tiuauial ihange in other groups, as shown by their known 

 pa Mintoio^x, i.s, jjioportioned to the change in their mode of 

 'to. tile I n-ulala exhibiting the greatest changes. 

 /iK' hxpnthcMs may be stated as follows : 



^ '^'■^'^'-'^ ancestois of the Tertiary mammals were 

 ""^^ '^i^iiii'ils ()(. vciy uniform skeletal characters, but 



' ^""^^^^'^^'t (httriontiated in dentition according as 

 iiiit. se^(U and nuts, oi insects, formed the staple of their 

 Ik -innini; ot the Mesozoic the available modes of 

 toi land veitebiates were chiefly the amphibious-aquatic, 



