628 RODENTIA. 



gated and the angular process of the jaw is well developed. 

 There are never more than three papillae circumvallatae on the 

 tongue. 



The rodents are usually small animals, though in a few 

 cases, e.g. the capybara, they may attain to some size. They 

 have fur which is often fine, but in some it may be modified 

 into spines on the back. The limbs are usually pentadactyle. 

 They are plantigrade or semiplantigrade and the nails on 

 the digits are usually claw-like, but in a few cases they have 

 the form of hoofs. 



The dentition is their most characteristic feature. Canines 

 are always absent and the lower jaw has never more than one 

 pair of incisors. The upper jaw also has only two incisors 

 except in the Duplicidentata (hares, rabbits and pikas), in 

 which there is a second pair of small incisors behind the large 

 pair. There is always a wide diastema behind the incisors. 

 The grinding teeth vary from f in the Hydromys to -| in the 

 rabbit. Three pairs of these are molars, the rest premolars. 

 If there are only three pairs of grinders or less than three 

 there are no premolars. In the majority the grinding teeth 

 are four pairs, p J- m -|. The premolars usually displace 

 milk molars. The milk dentition is varied. In some forms 

 with three pairs (e.g. Muridae] or less than three pairs 

 (Hydromys} of grinders, there appear to be no milk teeth at 

 all, and the dentition is monophyodont. In some forms 

 (e.g. Cavia) milk grinders are present in the embryo, but 

 absorbed before birth ; in some (e.g. Castor) the milk grinders 

 persist until the animal is half-grown ; and there are condi- 

 tions intermediate between these two. Milk incisors seem 

 generally to be absent. They have however been detected in a 

 few forms ; e.g. three pairs have been detected in the squirrel 

 as vestiges, while in the rabbit there appear to be two pairs 

 of deciduous incisors in the upper jaw. and one pair in the 



welt, Mem. Acad. Imp. Petersbourg, 1855. Forsyth Major, Nageriiber- 

 reste aus Suddeutschland u. der Schweiz, Palaeontographica, 22, 1873, 

 p. 75. Alston, Class, of the order Glires,P.Z.S., 1876, p. 61. Cowes and 

 Allen, Monograph of N. American Rodentia, U. S. Geol. Surv. Territ., 

 11, 1877. Schlosser, Die Nager der Europ. Tert., Palaeontographica, 

 31, 1884-5. Winge, Rodentia fra Lagos Santa, Brazil, Mus. Lund. 

 3, 1887. Thomas, Genera of Rodents, P.Z.S., 1896, p. 1012. Tullberg, 

 Das System d. Nagerthiere, Act. Ak. Upsala, 1890. 



