240 THE ETHIOPIAN REGION. [CHAP. 



Somaliland, which may be regarded as degraded descendants from 

 that type. The family is unknown either in Madagascar or in a 

 fossil state in Europe, although an extinct species of Rhizomys 

 occurs in the Indian Pliocene, and it is, therefore, in all proba- 

 bility a comparatively late immigrant into Ethiopia. In addition 

 to one genus ranging from Nubia to Siberia, the jerboa-family 

 (DipodidcE) has one peculiar Ethiopian genus, represented solely 

 by the Cape jumping-hare (Pedetes) ; a form so different from all 

 the others that it must constitute a sub-family apart. With this 

 genus we leave the mouse-like, and come to the porcupine-like 

 group of the rodent order, in which the family Octodontidce. has 

 nearly all of its representatives which are not Neogaeic confined to 

 this region, although the gundi (Ctenodactylus) of North Africa, in 

 the neighbourhood of Tripoli, is south Holarctic. This genus and 

 an allied type (Pectinator) from Somaliland, together with the next 

 form, alone represent a sub-family typically characterised by the 

 presence of a horny comb-like appendage and stiff bristles on each 

 of the hind feet ; Pectinator thus being Ethiopian. The South 

 African rock-rat (Petromys), although now included in the same 

 group, approximates to a sub-family of which all the members are 

 South American ; the resemblance between one of the latter and 

 the Ethiopian species being curiously close. The two species of 

 cane-rat (Triaulacodus 1 ) constitute the sole African representatives 

 of a sub-family containing a large number of Neogseic genera: 

 Probably, as there has already been occasion to remark, both the 

 Neogaeic and Ethiopian representatives of this family trace their 

 origin to the extinct Theridomyidce of the Oligocene of the Hoi- 

 arctic region or some nearly allied forms ; and as certain forms 

 occur in the Santa Cruz beds of Patagonia, it is probable that the 

 migration into Ethiopia was at least as early as that of the early 

 lemuroids, the extinct Pellegrinia of the Sicilian Pliocene being 

 the last survivor of the group in the northern half of the Old 

 World 2 . Although there is no generic type of porcupine (Hystri- 



1 This name is proposed to replace Aulacodus, Temm. (1827), which was 

 preoccupied in 1822 by Eschscholtz for a genus of Coleoptera. 



2 There is difficulty in this respect on account of the absence of hystrico- 

 morphous rodents from Madagascar; but perhaps the early forms entered the 

 west side of the continent, while the lemurs travelled in along the east. 



