

96 MAMMALIA. 



in which several of them reside tog-ether, passing through a hole in the 

 bottom, for the roots of the acorus on which they feed. They neither dive 

 nor swim well. It is this habit of building- which has induced some authors 

 to refer the Ondatra to the genus Castor. The second subdivision is that of 



ARVICOLA, Cuv. HTPUDJETJS, Illig. 



Our common Field Rats, which have a hairy tail, about the length of the 

 body, and simple or not palmated feet. 



<d. arvalis. (The Campagnol. ) Size of a Mouse; of a reddish-ash colour; 

 tail not so long as the body. It inhabits holes which it excavates in the 

 earth, where it collects grain for the winter. The multiplication of this 

 animal is sometimes so excessive as to cause much injury. 



GEOB.YCHUS, Illig. 



Or the Lemmings, Cuv. have very short ears and tail, and the toes of the 

 fore feet peculiarly well formed for digging. 



G. lemmus. (The Lemming.) A northern species, as large as a Rat, 

 with black and yellow fur, very celebrated for its occasional migrations in 

 innumerable bodies. At these periods they are said to march in a straight 

 line, regardless of rivers or mountains; and while no obstacle can impede 

 their progress, they devastate the country through which they pass. Their 

 usual residence appears to be the shores of the Arctic ocean. 



G. hudsonius; Mus. hudsonius, Gm., Schreb. (The Lemming of Hudson's 

 Bay.) A light pearly-ash colour; without tail or external ears; the two mid- 

 dle toes of the fore foot of the male seem to have double claws, which is 

 owing to the skin at the end of the toe being callous and projecting from 

 under the nail, a deposition of the part hitherto unknown, except in this 

 animal. It is the size of a Rat, and lives under ground, in North America. 



OTOMTS, Fred. Cuv. 



The Otomys are nearly allied to the Field Rats, and have also three grin- 

 ders, but they are composed of slightly arcuated laminae arranged in file. 

 Their incisors are grooved with a longitudinal furrow, and the tail is hairy, 

 as well as the ears, which are large. 



0. capensis, Fred. Cuv. (The Cape Otomys. ) Size of a Rat; fur marked 

 with black and fawn coloured rings; tail a third shorter than the body. 



DIPTJS, Gm. 



The Jerboas have nearly the same kind of teeth as the true Rats, except 

 that there is sometimes a very small one immediately before the upper mo- 

 lars. The tail is long and tufted at the end; the head large; the eyes large 

 and prominent; but their principal character consists in their posterior ex- 

 tremities, which, in comparison with the anterior, are of a most immode- 

 rate length, and above all, in the metatarsus of the three middle toes, which 

 is formed of one single bone, resembling what is called the tarsus in Birds, 



