RODENTS OF LIBYA 105 



nomic relationships among these scattered populations, and when the 

 intervening areas are represented by specimens, they will probably 

 contain intergrades. A clinal gradient of progressive increase in size 

 probably will be demonstrable from east to west. 



In most characters, gerbils from Bzema Oasis are referable to 

 G. g. aeruginosus, but in greatest length of skull and dorsal color, 

 they show intergradation with Gerbillus gerbillus psammophilous to 

 the north. The nearest populations of G. g. psammophilous which 

 could provide the gene pool causing these intermediate characters 

 occur in the vicinity of Bir Bu Zarregh and Bir el Harasc north of the 

 Sand Sea of Rebianna. Apparently, then, this sand sea is not an 

 absolute barrier but a filter for genetic exchange between animals 

 strictly referable to G. g. psammophilous and G. g. aeruginosus. 



Animals belonging to G. g. aeruginosus are exceptional in their 

 wide range of dorsal coloration, and three distinct groups are recog- 

 nizable, ranging from Ochraceous-Buff, to Clay Color, to tawny-rust. 

 The majority of individuals are tawny-rust. A genetic response to the 

 local character and color of the sand in Cufra Oasis may account for 

 this wide range of dorsal coloration. 



One specimen, 319683, from El Giof, Cufra Oasis, differs markedly 

 from all other specimens of G. g. aeruginosus in having a more promi- 

 nently vaulted skull (especially the parietals), smaller and less inflated 

 auditory bullae, and heavier zygomata. In all other morphological 

 characters this specimen clearly represents Gerbillus gerbillus. In the 

 marked inflation of the braincase, however, it resembles Gerbillus 

 eatoni whose range includes the Egyptian and Libyan coasts. This 

 similarity to G. eatoni is entirely fortuitous, and this specimen is 

 probably aberrant or represents an extreme in the range of variation 

 for the subspecies. 



Ecological observations. These gerbils prefer the sandy areas 

 along the periphery of the oasis but occasionally were taken from 

 areas of open sand within the palm groves. At El Giof, a large series 

 was obtained from a sandy clearing in the interior of the oasis. Usually 

 however, Gerbillus pyramidum and Gerbillus campestris are more 

 abundant in the oasis proper. 



At Bzema, El Giof, El Hauuari, and most of the other large oases 

 of southern Cyrenaica, large saline lakes occupy the interior of the 

 oasis and form hard precipitates of salt along their margins. These 

 barren areas are surrounded by a ring of sedges and halophytic plants, 

 and G. gerbillus occurs here only sparingly; however, Gerbillus cam- 

 pestris occurs abundantly in these mesic habitats. 



The name aeruginosus, from the Latin meaning rust or rusty, 

 alludes to the rusty dorsal color of most specimens. 



285-134 O — 68 8 



