156 NATURAL HISTORY. 



when the skull is looked at from the side. The lower jaw in its characters presents some resemblance 

 to the same part in the Leporidse ; but it has the condyle for its articulation with the skull trans- 

 verse, and fitting into a cavity of corresponding direction, a character which occurs in no other 

 Rodent. Of the remainder of the skeleton we need only state that the animal possessed perfect 

 clavicles ; that the shoulder-blade and humerus somewhat resemble those of the Beaver ; that the 

 fibula articulated with the heel-bone ; and that both front and hind limbs possessed five toes, some of 

 which, judging from the form of the terminal joint, were probably furnished with hoof-like claws. 



Thus, as regards its affinities in the order Rodentia, Mesotherium presents resemblances in its 

 lower jaw (as also in some peculiarities of the skull), and in the articulation of the heel with the 

 shank, to the Hares ; while in the shortness of the incisors and some other cranial peculiarities, the 

 form of the shoulder-blades, and the probably hoof-like character of the claws, we may notice an 

 approach to the Cavies, which are also South American forms, and especially to the Capybara, which 

 it probably resembled in its habits, although, if the evidence of the Beaver-like shoulder-blade and 

 humerus be taken into account, it would appear to have been still more aquatic. 



On the other hand, the resemblance to certain other Mammalia, and especially to some aberrant 

 Ungulates, is unmistakable. The number of incisor teeth is the same as in Hyrax, and in these 

 teeth there is also a certain amount of resemblance to the curious genus Toxodon, in which the 

 incisors are four in the upper and six in the lower jaw, and worn away in somewhat the same fashion. 

 In Toxodon also, the convexity of the curve of the molars is turned outwards. Certain other 

 characters of Mesotherium such as the mode of articulation of the lower jaw, and the peculiar 

 connection of some of the caudal vertebrse with the ischiatic bones present resemblances to the 

 Edentata. As Mr. Alston says, " It appears to have been a survivor, to Pliocene times, of a much 

 earlier type, which represented an era at which the Rodents were not yet clearly marked off from 

 their allies. In fact, Mesotherium seems to continue into the order Glires that line of affinity which 

 Professor Flower has pointed out as extending from the typical Ungulates through Hyracodon, 

 HomalodontotJierium, Nesodon, and Toxodon." 



The general relationships of Mesotherium to the other Rodents, and of these among themselves, 

 are represented by Mr. Alston in a diagrammatic form, from which the following scheme, which will 

 serve also as a table of the families, is derived : 



< 4. Haplodontida;. 



EH 1. SCIUROMORPHA. 1. Anomaluridcc. \ <j3 



^ 5. Castoridce. 2. Sc'mridce. 



2; 3. Ischyromyidce. 



y* 



6. Myoxidce. 7. Lophiomyidce. 



8. Mitridcc. 

 10. Geomyidce. 11. Theridomyida. 



2. MYOMORPHA. 

 9. Spalacida. 12. Dipodidcc, 



14. Hystricida. 13. Octodontidcc. 



3. HYSTRICOMORPHA. 



16. Dasyproctida. 15. Chinchillidce. 



20. Leporidce. 



17. Dinomyidce. 

 19 Lagomyidce. 18. Caviidte. I t-5 



III. RODENTIA HEBETIDENTATA. 



21. Mesotheriidce. 



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 

 SciuridcK, Castoridce, Myoxidce, Muridce, Geomyidce, Chinchillidce, and Leporidce, already differen- 



