MAMi.lALIA. 491 



Genus 2. — Fibeb. — Cuyj'er. 



Generic Character. — Incisors 2^ no canines, grinders |-^ ; 

 total 16. Lower incisors sharp-pointed, and convex in front ; 

 grinders with flat crowns, furnished with scaly transverse zigzag 

 laminae ; fore feet with four toes, and the rudiments of a thumb ; 

 hind feet with five toes, having the edges furnished with stiff 

 hairs, used in swimming like the membrane of palmated feet ; 

 tail long, compressed laterally ; both sexes secrete an odorifer- 

 ous musky unguent. 



Fiber Zibethicus. — The Musk Rat, or Ondatra. — Plate 

 XV. fig. 2 — Described, vol. II. p. 336, &c. 



Genus 3. — Arvicola. — Desmarest. 



Generic Character. — Incisory teeth % no canines, grinders '^^ j 

 total ]6. Grinders are flat on the crowns, with angular plates 

 of enamel ; ears large ; anterior toes with nails ; tail round, 

 covered with hair, and almost as long as the body. 



Arvicola amphibius. — The Water Rat Plate XV*. fig. 4. 



— Fur blackish gray, slightly tinged with yellow above, lighter 

 underneath ; tail black. Somewhat larger than the common 

 Norway rat. Inhabits Europe, Northern Asia, and North 

 America. 



One variety of this species is entirely black, another with a 

 white spot on the shoulders, and a third black with white feet. 



Genus 4- — Isodon. — Say. 



Generic Character. — Incisors*, no canines, grinders |-| ; to- 

 tal 20. Grinders converging, with flat crojvns not divided into 

 radicles ; the incisory teeth not narrowed at the top, but obtuse- 

 ly rounded ; fore feet four-toed, with a small tubercle in place 

 of a thumb; hind feet four-toed ; the toes in all the feet divided, 

 and rest equally on the sole in walking. 



Isodon pilorides. — Known only from the skeleton. 



Genus 5. — Neotoma. — Say and Ord. 



Generic Character. — Incisors I, molars ^-J : total 16. The 

 incisors in the upper jaw slightly rounded on their anterior face ; 

 molars with profound radicles ; first molar with five triangles, 

 one of which is anterior, two exterior, and two interior ; second 

 molar with four triangles, one anterior, two on the exterior side, 



2t 2 



