92 U.S. NATIONAL MUSEUM BULLETIN 2 75 



ularly as concerns the size and shape of the auditory bullae, the 

 relative length and width of the anterior palatine foramina, the 

 degree of arching of the braincase, the length of the molariform 

 too throw, and the size of the individual molariform teeth. This poly- 

 morphism suggests the presence of sibling species within this popula- 

 tion from Nalut, but overlap of characters occurs too frequently, 

 and no specific combinations of characters are demonstrable for 

 individual specimens. Apparently this population from Nalut repre- 

 sents one of pronounced genetic fluidity which has engendered this 

 wide array of morphological patterns. 



Although this new subspecies is known from only the type locality, 

 its range doubtless includes the coastal plain and littoral deserts of 

 southern Tunisia and, in Libya, probably extends much farther 

 along the coastal plain to the east. In Libya, there are no apparent 

 barriers to dispersal along the coastal plain, and intergradation 

 between animals of G. a. nakitensis and those of the nominate sub- 

 species takes place probably somewhere between Tripoli and Nalut. 



Ecological observations. The type locality consists of a series 

 of vegetated dunes. These dunes are not extensive and occur sporadi- 

 cally as irregular bands along the coastal plain near the coastal 

 escarpment. The role of the coastal escarpment in retarding the 

 velocity of the winds probably has resulted in the formation of these 

 coastal dunes. 



Forty-seven gerbils and five jirds were obtained from a single night 

 of trapping at 40 kilometers east-northeast of Nalut. The presence 

 of numerous active burrows plus this large catch suggests a high 

 population density of rodents. A severe wind storm or "ghibli" 



10 2 10 4 10 6 10 8 11 11.2 114 116 



Figure 17. — Statistical comparison of length of auditory bulla of the subspecies of Gerbillus 

 aureus: A, G. a. aureus; B, G. a. favillus; C, G. a. nalulensis. 



