18/6.] 



MR. E. R. ALSTON ON THE ORDER GLIRKS. 



6h 



here claimed for all the families and subfamilies. Such is the variety 

 of the extent of differentiation that it appears to me that no Procrus- 

 tean standard can be applied. Either we must load our memories 

 with tribes, legions, cohorts, series, superfomilies, &c., or we must be 

 content with divisions pretending only to an approximate equality of 

 value. 



General Remarks. 



The first suborder of Rodents, Glires Simplicidentati, con- 

 tains 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*. Tiie 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 de- 

 scending into the scrotal pouchest. 



Of this suborder the first section, Sciuromorpha, has for con- 

 stant characters the combination of a peculiar form of mandible with 



Fig. 1 . 



Mandible of Arctomi/s marmotta. 



the persistence of the fibula as a distinct bone throughout life. The 

 former character at once separates it from the Hystriconiorpha, 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. Owen, Anat. of Vert. iii. p. 640. 



Proc. Zool. Soc— 187ti, No. V. 5 



