II.--RODENTIA DUPLICIDENTATA. 4. _Haplodontidæ_. { I. 1. SCIUROMORPHA. 1. _Anomaluridæ_. { -- S 5. _Castoridæ_. 2. _Sciueridæ_. { R I 3. _Ischyromyidæ_. { O M | { D P { E L 6. _Myoxidæ_. 7. _Lophiomyidæ_. { N I 8. _Muridæ_. { T C 10. _Geomyidæ_. 11. _Theridomyidæ_. { I I 2. MYOMORPHA. { A D II.--RODENTIA 9. _Spalacidæ_. 12. _Dipodidæ_. { E DUPLICIDENTATA. | { N | 14. _Hystricidæ_. 13. _Octodontidæ_. { T | 3. HYSTRICOMORPHA. { A /----/ \----\ 16. _Dasyproctidæ_. 15. _Chinchillidæ_. { T 20. _Leporidæ_. { A. 17. _Dinomyidæ_. { 19 _Lagomyidæ_. 18. _Caviidæ_. { | | III.--RODENTIA HEBETIDENTATA. 21. _Mesotheriidæ_. It seems quite clear, even from the above brief sketch of the history of the Rodentia in time, that, except in the case of _Mesotherium_, the fossil remains of animals belonging to this order furnish us with no important information bearing upon their alliances and possible origin. They make their earliest appearances, so far as we know, in deposits of Eocene age; and the earliest forms the remains of which are sufficient to give us a clear insight into their nature, are manifestly members of families, and often nearly allied to species still extant in the regions where their traces are now found. Thus in Eocene and Miocene deposits, we have representatives of the families _Sciuridæ_, _Castoridæ_, _Myoxidæ_, _Muridæ_, _Geomyidæ_, _Chinchillidæ_, and _Leporidæ_, already differentiated as at the present day, so far as the evidence goes; and it is clear that we must go much further back in time to seek the earliest appearance of the Rodent type, whether it branched off directly from the Marsupial series, or passed, as would seem to be indicated by _Mesotherium_, through a sequence of forms more or less related to the Ungulates.