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Show 74 MR. E. R. ALSTON ON THE ORDER GLIRES. [Jan. 18, the order), their number in the lower jaw (as in Hyrax), the curvature of the molars, of which the convexity is not inwards but outwards (as in Toxodon), the transverse form of the condyle of the mandible and the glenoid fossa, and the articulation of the ischia with some of the caudal vertebrae (as in some Edentates). With regard to the first of these characters, we have seen that the enamel is present, though very thin, on the back as well as the front of the incisors of the Glires duplicidentati; and a side view of these teeth in the Hares and in some of the lower Hystricomorpha shows a sort of gradation between the acute edge of the more highly specialized forms and the hollowed crown of Mesotherium. Of the condyle and glenoid cavity also it is to be noted that, although they are not transverse in any existing Rodent, yet their shape is much less clearly defined in the less-specialized forms. In other respects the whole skeleton of Mesotherium presents so many resemblances to the Rodents that it seems to m e that we must follow Professor Gervais in placing it in that order rather than in any other. Its affinities with the more aberrant Ungulates, and especially with Toxodon, cannot, however, be overlooked ; and 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 Prof. Flower has pointed out as extending from the typical Ungulates through Hyracodon, Homalodontotherium, Nesodon, and Toxodonf. As to the affinities of Mesotherium within the Order Glires, they do not appear so exclusively leporine to m e as to M . Gervais. While agreeing with the Hares in many important points, as in the form of the mandible (which, however, is still more like that of Hyrax), in that of the brain, as indicated by a cast of the cranial cavity, and in the articulation of the fibula with the calcaneum, Mesotherium rather inclines in other particulars to the Glires simplicidentati, and especially to Hydrochcerus. Among these may be enumerated the comparative shortness of the incisors, the smallness of the incisive foramina, the development of the bony palate and of the paroccipital processes, the depth of the malar, the form of the scapula, &c. Some at least of the toes seem to have been subungulate; and tbe terminal phalanx figured by Gervais (pl. xxv. fig. 34) very closely resembles the same bone in the Capybara. As Mesotherium thus appears to present relationships to each of the existing suborders, combined with peculiarities which forbid its admission into either, I venture to propose the establishment of a third for its reception. It might be named G L I R E S H E B E T I D E N - TATIJ, and characterized by the incisors being two above and four below, the molars curved inwards and the condyles placed transversely. Before ending this paper, with a Table of the characters of the various divisions here adopted, attention may be drawn to the accom- * W e have seen that in Europe the Rodents were fully differentiated in the Eocene period (antea, p. 70). t Phil. Trans. 1874, p. 181. % Hebes -tis, blunt; dentatus, toothed. |