164 MATTHEW— RECENT DISCOVERIES OF 



same type of triangular tusks and is of quite moderate size. Its 

 nearer affinities, however, are with the great extinct ground sloths. 

 It belongs to the same family as Megalonyx of the North American 

 Pleistocene, but is not very closely related to it, and is very much 

 smaller and in some respects more primitive. Although thus related 

 to a North American genus of ground sloths Acratocnus points to 

 a South American origin, for Megalonyx is known to be an immi- 

 grant type in the north, derived from South American ancestors in 

 the Miocene. 



The rodents all belong to the Hystricomorph or porcupine sec- 

 tion of this order, which has its headquarters in South America. 



The largest of them, Elasmodontomys, is closely related to Ajh- 

 hlyrhisa, the gigantic extinct rodent of the island of Anguilla. 

 Another smaller genus, Heptaxodon, also belongs in this neighbor- 

 hood, but is less closely related. These are put into the family 

 Chinchillidse, but their nearest affinities are with the extinct Mega- 

 mys and Tetrastylus of the Pliocene of Argentina. 



A second group of rodents, Heferopsomys and Homopsomys, is 

 related, but rather distantly, either to the living agouti, Dasyprocta, 

 or the spiny rats of South America ; it has also, as will appear, some 

 extinct relatives in Cuba and Hayti. 



A third extinct rodent from Porto Rico, Isolohodon, is nearly 

 related to Plagiodontia, 3.n eximct or almost extinct rodent of Hayti, 

 and more distantly to the hutias (Capromys) still living in Cuba 

 and Jamaica. Like the preceding group it is rather distantly related 

 to any continental rodents ; it is referred to the Octodontidae. 



That is the full extent of the mammal fauna of Porto Rico. Its 

 affinities are chiefly South American, but distant, pointing to long 

 isolation. The near relationship to the extinct Anguillan rodent is 

 significant. The bank of shallow water which extends eastward 

 from Porto Rico includes the Virgin Islands; and Anguilla with the 

 adjoining islands also stands on a shallow bank of considerable 

 extent. Vaughan has shown the probability of part or all of these 

 banks being above water in the Pleistocene. But between the Virgin 

 Islands and the Anguilla group there is a narrow trench of very 

 deep water. Possibly this trench is due to faulting of recent origin 

 and the islands were connected in the Pliocene or Pleistocene. The 



