74 MR. E. R. ALSTON O.N THE ORDER GLIRES. [Jan. 18, 



the order), their number in the lower jaw (as in Hyrax), the curva- 

 ture of the molars, of which the convexity is not inwards but out- 

 wards (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 Me&otheriurn. 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 Ro- 

 dents that it seems to me that we must follow Professor Gervais in 

 placing it in that order rather than in any other. Its aflSnities with 

 the more aberrant Ungulates, and especially with Toxodon, cannot, 

 liowever, 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 Toxodon-^. 



As to the affinities of Mesotherium within the Order Glires, they 

 do not appear so exclusively leporine to me 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 hke 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, Jind 

 ■ especially to Hydrocharus. 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 the 

 terminal phalanx figured by Geivais (pi. 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 Glires hebetiden- 

 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 

 variX)us divisions here adopted, attention may be drawn to the accom- 



* We have seen that in Europe the Rodents were fully differentiated in the 

 Eocene period (antea, p. 70). 



+ Phil. Trans. 1874, p. 181. \ Hebcs -tis, blunt ; dentatus, toothed. 



