ORDER RODENTIA. 1415 



deposits are Epiblema and Tetrastylus ; one species of the latter 

 having been originally referred to Theridomys. 



Family Hystricid^l. — The Porcupines are well characterised by 

 their spiny covering. The number of their cheek-teeth is the same 

 as in the preceding families ; and these teeth have both external 

 and internal enamel-folds. An extinct species of the American 

 genus Syncetheres occurs in the Pleistocene of the Brazilian caves, 

 while a species of the other American genus Erithizon is recorded 

 from cave-deposits in Pennsylvania. Atherura is found in the 

 Pleistocene of Southern India. Hystrix occurs in the Pleistocene 

 and Pliocene of India; in Europe from the Upper Pliocene of the Val 

 d'Arno down to the Middle Miocene, and perhaps to the Quercy 

 Phosphorites ; while in North America it is represented both in the 

 Pliocene and the White River Miocene. Finally, Mylagaulus from 

 the Miocene of the United States may be allied either to Hystrix or 

 Dasyprocta (Agouti). 



Family Castoroidid^:. — The gigantic Castoroides, of the Pleis- 

 tocene of the United States, is now generally regarded as entitled to 

 represent a distinct family although originally placed in the Cas- 

 toridce. Although presenting certain cranial features recalling Castor, 

 its dentition comes nearest to that of Chinchilla and Hydrochcerus. 

 The extinct genera Loxomylus and Amblyrhiza, from the Pleistocene 

 of the Antilles, may probably be included in the same family ; their 

 dentition presenting many resemblances to that of Chinchilla. Cas- 

 toroides must have attained the dimensions of a Bear. 



Family Octodontid^:. — With the exception of Ctenodactylus the 

 number of the cheek-teeth in this Ethiopian and South American 

 family 1 is the same as in the Hystricidce; these teeth having both ex- 

 ternal and internal enamel-folds, with either imperfect or perfect roots. 

 In the South American Pleistocene we meet with existing and a few 

 extinct species of the genus Carterodo?i, which is characterised by 

 the broad and grooved incisors, and also of Myopotamus (Coypu), 

 Echinomys, Loncheres, Phyllomys, and Ctenomys. Other forms from 

 the South American Tertiaries allied to the latter have been named 

 Phloramys and Pithanotomys ; while Morenia and Orthomys, from 

 the infra-Pampean beds of Parana, and Tribodon, from Monte Her- 

 mosa, are regarded as related to Myopotamus. The extinct Pelle- 

 grinia, from the Sicilian Pleistocene, may be allied both to the 

 Ethiopian Ctenodactylus and the American Octodon. 



Family Theridomyid.e. — This extinct family appears to be most 

 nearly related to the preceding although connected by Archceomys 



1 3 



with the Chinchillidce. The cheek-teeth number Pm. -, M. — . In 



1 3 



1 One species of Echinomys occurs in Central America. 

 VOL. II. 2 L 



