These differences appear to place the Lagomorphi at a much greater distance from the group made up of the Hystricomorphi, the Myomorphi, and the Sciuromorphi, than any one of those sections is from any other. Of them the Hystricomorphi, especially as represented by the Cavies, come nearest to the Lagomorphi; the Myomorphi should be placed centrally, and the Sciuromorphi highest in position.
Mr. E. R. Alston, P. Z. S. Jan. 1876, pp. 73, 74, has proposed the establishment of a third Suborder, that of Glires hebetidentati, for the reception of the genus Mesotherium found in South American Pliocene deposits, and represented by an animal a little larger than the Capybara. This animal had the same number of incisor teeth placed in the same way, viz. two above and four below, as the Hyrax; they were surrounded, not merely faced, with enamel, and were not brought to a sharp cutting edge by wear, but came to present a transversely hollow blunt surface, whence the name hebetidentati. The mandibular condyle and the glenoid fossa were not specialized to perform the antero-posterior movements so characteristic of the Simplicidentati, and the three anterior and upper molar teeth were convex outwards 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 Sciuromorfihi 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 Pl. 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 Cyclothurus 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, lxv. 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 ambiguity que nous offre, en effet, ce singulier animal, ressemblant 1°. aux Rongeurs par la disposition de ces incisives, du mesodonte (p. 145), et par les dents uni-radiculees; 20. aux jeunes Pachydermes par la forme generale, et le rudiment des fossettes des incisives et des molaires; 30. aux Edente'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 40. aux Cetace's, d'une part, par l'enfoncement de l'occiput, l'affaissement de la voute du crane et la petitesse de Pencephale, qui en est la condition premiere, et d'autre part, par le nez large et court, un peu ouvert en dessus ce qui Concorde avec l'ide'e de M. le Dr. Se'ne'chal qui pense que le Mesotherium etait, peut-etre, un animal aquatique; neanmoins, au milieu de ces conformites 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 intermediaire qui les relie.
Cet anneau serait-il, selon la pense'e de Blainville, un des chainons perdus de la serie animale?'