AFFINITIES OF RODENTS. 45 



wards as in Toxodon, not inwards as in typical Rodents. The massive heavy 

 ridges on the skull give it an appearance not unlike that of the Sirenian skull when 

 looked at from above ; in some particulars, such as the great development of its 

 paroccipitals and its palate, it resembles at once the Capybara, an exclusively South 

 American type, and the Artiodactyla, but notably differs from both, and resembles 

 the Sciuromorphi or most specialized Rodents, in possessing perfect clavicles. 

 Professor Gervais, who has figured its skull and skeletal bones in his Zoologie et 

 Palaeontologie Ge'ne'rales, pp. 134-139, 145, and PI. xxii-xxv, considers its affinities 

 to be Lagomorphous ; and it resembles the Hares and differs from the Cavina in 

 having its fibula in articular relation with the calcaneum. But it differs from all 

 existing Rodents in having its intermaxillaries so short as to allow the maxillaries 

 to abut upon the nasals ; in having the cutting surfaces of its incisors pit-shaped 

 like those of the Horse ; and- in having, as have Perissodactyla, Suidae, and Pro- 

 boscidea, the two halves of the mandible anchylosed, not merely suturally joined at 

 the symphysis. And with its other characteristics, not found to coexist in living 

 animals, it combines the peculiarity, observable in the South American Bruta, 

 except Cydothurus didactylus, of having the ischium articulated with the caudal 

 vertebrae. 



In face of these anatomical facts recovered from the fossil remains, and 

 bearing in mind that with them may have been combined in the living animal 

 differences in the soft parts which would have definitely prevented us from ranking 

 it with the Rodentia, we may hesitate to accept Mesotherium as constituting a third 

 suborder of that division of Mammals. Its various affinities appear to be fairly 

 stated by M. Serres in his fifth Memoir relating to it, Comptes Rendus, Ixv. 1867, 

 p. 599, in a passage which, for this reason, as also because it shows on how many 

 sides orders, which may at first sight appear to be sharply circumscribed, may come 

 into relation with each other, it may be well to give in extenso : ' Quelque am- 

 biguitd que nous offre, en effet, ce singulier animal, ressemblant i. aux Rongeurs 

 par la disposition de ces incisives, du me*sodonte (p. 145^ et par les dents uni- 

 radiculees ; 2. aux jeunes Pachydermes par la forme ge'ne'rale, et le rudiment des 

 fossettes des incisives et des molaires ; 3. aux Edent^s, ses contemporains, par la 

 masse, la lourdain de sa tete et de ses membres, ainsi que par la bifurcation de la 

 derniere phalange, enfin 4. aux Cetacds, d'une part, par 1'enfoncement de 1'occiput, 

 1'affaissement de la voute du crane et la petitesse de 1'encephale, qui en est la con- 

 dition premiere, et d'autre part, par le nez large et court, un peu ouvert en dessus 

 ce qui concorde avec Pide'e de M. le Dr. Sene'chal qui pense que le Mesotherium 

 e*tait, peut-etre, un animal aquatique ; ne"anmoins, au milieu de ces conformity's si 

 diverses, celles qui le rapprochent plus particulierement des Rongeurs et des 

 Pachydermes dominent tellement les autres, que c'est entre ces deux ordres de 

 Mammiferes que nous croyons qu'il doit etre place* comme un anneau interme'diaire 

 qui les relie. Get anneau serait-il, selon la pense'e de Blainville, un des chamons 

 perdus de la seVie animale?' 



