50 The Rodentia in Evolution. 



absolutely eosmopolitan. Their osteological characters are, 

 a narrow frontal and parietal region (cf. fig. 3), slender 

 zygomatic arch, malar bone slender and very short (fallen out 

 in specimen figured) molars never 5 usually 4 often 3 or even 

 2, generally 4 and sometimes 5 sacral vertebrae, in some no 

 pulie symphisis (cf. fig. 9 Geomvs or pocket gopher.) The 

 fibula is always anchylosed to the tibia and the toes are 4 

 or 3, in some the molars are found to found to form a canon 

 bone. A few among the many mice are mentioned to convey 

 an idea of the great range of habit within the limits of this 

 exceedingly clearly defined sub order. 1, The water rat, 

 Hydromys is nearly perfectly aquatic, with close glossy fur, 

 flat head, slender body, a long tail, partially webbed feet, a 

 native of Van Diemensland. 2, Gerhilles of which 50 species 

 are said to be known is a leaping rodent with elongate leg 

 and tail. 3, The Hamster mouse is a burrowing rodent 

 with large cheek pouches as in the very different pocket 

 gopher, tail very short but eyes large (unusual in fossorial 

 animals) and toes short clawed. 4,^ The house mouse also 

 Acomys, a mouse with spines in the skin recalling the porcu- 

 pine belongs here. 5, The field mouse is a burrowing or run- 

 ning rodent, closely allied to it are the arctic ^Cuniculus and 

 lemming and the muskrat an aquatic mammal with slightly 

 webbed feet and horizontal flattened scaly tail. 6, The 

 pouched gophers are considered a separate family of myo- 

 morphs but they are very like, in many respects, the field 

 mice from which they seem to have been descended. They 

 are chiefly fossorial, have minute eyes, short tail, a very 

 remarkable sacrum and pelvis which has no pubic symphesis. 



7, Still more extremely fossorial are the rodent moles, Siph- 

 neae, which are perfectly subterranean, have no functional 

 eyes, no external ear and limbs short, stout and mole like. 



8, The Jerboa or jumping mouse, or kangaroo-rat is a slender 

 mouse like rodent with long slender hind limbs and reduced 

 fore limbs, anterior part of body the cervical vertebrae being 

 anchylosed, hind foot of only 3 toes and the metatarses are 

 fused. The mice present the appearance of a very productive 

 race of beings forced by pressure of numbers and competition 

 in various forms to push out from terrestrial habits to fos- 

 sorial, aquatic and other ones and as if they had some of 



