484 RODENTIA 



Carterodon. 1 — This genus, which was originally described upon 

 the evidence of skulls from the Brazil caves, but subsequently found 

 living, is readily distinguished by the broad and grooved incisors. 

 The upper molars have one inner and two outer enamel-folds ; 

 those of the lower jaw being the reverse of this. 



Fossil Forms. — Remains of the existing genus Loncheres occur in 

 the Brazilian cave-deposits, which also yield the extinct Dicolpomys. 

 A large number of fossil Ododontidce from the Tertiaries of South 

 America have been described under many generic names, but it 

 will be sufficient to mention that PMoramys and Pithanotomys are 

 considered to be allied to Ctenomys; while Morenia, Orthomys, and 

 Trilodon show affinity to Myopotamus. Pellegrinia, from the Pleisto- 

 cene of Sicily, may be allied both to Ctenodactylus and Octodon. 



Family Theridomyid.e. 



This extinct family, which is represented in the Tertiaries of 

 Europe and the United States, comprises several genera of com- 

 paratively small Rodents, which are regarded by Dr. Schlosser as 

 nearly related to the Ododontidce, although connected by Archceomys 

 with the ChinchUlidce. The dental formula is the same as in the 

 Ododontidce. In the typical genus Theridomys, from the Lower 

 Miocene and Upper Eocene of Europe, the molars are rooted, and 

 have three or four re-entering enamel-folds, which form isolated 

 discs on the worn crowns. Syllophodus, from the Miocene of the 

 United States, is closely allied. Protechinomys and Trechomys are 

 genera from the Phosphorites of Central France with rooted molars ; 

 while in Archceomys of the same deposits the molars are rootless, 

 with the enamel-folds dividing their crowns into laminae, as in the 

 Chinchillas. 



Family Hystricid^e. 



Build stout. Limbs subequal. A number of long and stout 

 spines in the integument. Facial portion of skull short and broad, 

 and the jugal without an inferior angle. Molars with external and 

 internal enamel-folds ; completely or partly rooted. 



Subfamily Synetherinse. — Molars rooted; clavicles complete; 

 upper lip not cleft ; soles tuberculated ; pollex absent ; four mammae ; 

 tail generally prehensile ; spines mixed with long hairs. This group 

 is confined to America, all the forms except one being arboreal, 

 and their habits less strictly nocturnal than in the next subfamily. 

 There are three genera. 



Erethizon.' 2 — Represented by the common Canadian Porcupine 



1 Waterhouse, Nat. Hid. o/Mamm. vol. ii. p. 351 (1848). 

 - F. Cuvier, Dents des Mammiferes, p. 256 (1825). 



