AFFINITIES OF RODENTS. 39 



affinity which has so often been alluded to as existing between them and the 

 Ungulata into a clearer light than even the most detailed account of the anatomical 

 resemblances which exist between the living representatives of the two groups 

 under comparison. The most obvious, though perhaps also essentially the least 

 important difficulty besetting such a comparison, is that which is based on the 

 difference in size ; but this is done away with by the discovery in American 

 Miocene deposits of ' various small ruminant-like animals, some not larger than a 

 Squirrel in size, to which the names Leptomeryx, Hypisodus^ Hypertragulus, have 

 been applied.' See Flower, Proc. Royal Institution, March 10, 1876. On the 

 other hand, the discovery in the South American Pliocene deposit of the animal 

 known a*s Mesotherium^ which was a little larger than the Capybara, and has been 

 supposed to link the Rodents and notably the Leporidae to the Perissodactyle type 

 by many connecting peculiarities, whilst retaining itself so many of the character- 

 istics of the Rodent order as to have induced Mr. Alston, P. Z. S. 1876, pp. 73, 74, 

 to create for its reception a third suborder, that of Glires hebetidentati (see p. 44), 

 equivalent to each of the two other suborders, those of Glires dupliddentati and 

 Glires simpliddentati^ into which Rodents may be divided, might seem to give as 

 positive an illustration of the absence of any sharp circumscription in the delimita- 

 tion of this group as can be asked for. The order, Tillodontia, established by 

 Professor O. C. Marsh, would' seem to bear more striking evidence still in the 

 same direction, coming as it does from the earliest geological formation, the lower 

 Eocene, in which Rodents are found ; and combining some of the characters of 

 Carnivores with those of both Ungulates and Rodents, as though they were 

 survivors of some still earlier type unrepresented, as indeed are all Mammals, 

 in any as yet explored strata of the Cretaceous age. Similar affinity has often 

 been spoken of as existing between the Toxodontia of the Post-Tertiary South 

 American deposits and the Rodents; but as this much later date might have 

 led us to expect, the similarities between the two orders are by no means so 

 clearly made out. 



As regards existing Ungulata, the affinities of the Rodentia are distributed pretty 

 evenly between the two divisions of Ungulata Artiodactyla, the non-Ruminantia 

 s. Bunodonta and the Ruminantia s. Selenodonta (for which see W. Kowalewsky, 

 Ph. Tr. vol. 163, pp. 69-74, 1873) and the Perissodactyla ; but it must be 

 borne in mind that though the differences between the now existing Hoofed 

 animals thus named are very sharply defined, geological researches, especially 

 in the American Tertiaries, have revealed to us forms in which these lines of 

 demarcation tend to become obscured, or at least approximated. It would be 

 easy to say, for example, that the imperfection of the orbital ring was a point of 

 considerable importance, and united the Rodentia with the Perissodactyla; but 

 Professor O. C. Marsh (see Introduction and Succession of Vertebrate Life in 

 America, 1879, p. 161) states that several species of Cervidae from the lower 

 Pliocene of the west of America fail to have the orbit closed behind. 



A simple way of expressing the known facts may perhaps be furnished by 

 saying that though the Rodentia with reference merely to their claws would be 

 ranged with the Unguiculata rather than with the Ungulata of Linnaeus (see 

 Systema Naturae, ed. xiii. vol. i. p. 17), they nevertheless present both in their 

 skeleton and in their internal organs more marked points of essential affinity to 



