| OCR Text |
Show 1876.] MR. E. R. ALSTON ON THE ORDER GLIRES. 65 here claimed for all the families and subfamilies. Such is the variety of the extent of differentiation that it appears to me that no Procrustean standard can be applied. Either we must load our memories with tribes, legions, cohorts, series, superfamilies, &c, or we must be content with divisions pretending only to an approximate equality of value. General Remarks. The first suborder of Rodents, G L I R E S SIMPLICIDENTATI, contains an enormous majority of both the recent and extinct forms, and is at once proved by its dentition to be the most highly specialized division of the order. There is only one pair of incisors above and below at all ages ; and their enamel is restricted to their front surface. In the skull, the incisive foramina are moderate and separate, the optic foramina are very rarely confluent, and there is an alisphenoid canal*. The fibula is either ankylosed below to the tibia or free, and does not articulate with the calcanium. Vesicular glands are present; and the testes are usually abdominal, only temporarily descending into the scrotal pouchest. Of this suborder the first section, Sciuromorpha, has for constant characters the combination of a peculiar form of mandible with Fig. 1, Mandible of Arctomys marmotta. the persistence of the fibula as a distinct bone throughout life. former character at once separates it from the Hystricomorpha, the latter from the Myomorpha. In the mandible the angular portion springs from the lower edge of the bony covering of the inferior incisor, not from its outer side; and its outline is more or less rounded * Cf. Turner, P. Z. S. 1848, p. 65. t Cf. Oweu, Anat. of Vert. iii. p 649 PROC. ZOOL. Soc-1876, No. V. 5 |
