RODENTS OF LIBYA 229 



length of skull 33; condyloincisive length of skull 29.2; crown length 

 of upper molariform too throw 4.9; least interorbital breadth 11.6; 

 length of nasals 12.1; breadth of rostrum at level of antorbital foram- 

 ina 5.1; greatest breadth of braincase 16.9; greatest breadth across 

 antorbital processes 22. 



Diagnosis. Upperparts Cinnamon-Buff grading to Light Ochraceous 

 Buff in interauricular and supraorbital regions; rump decidedly darker 

 owing to greater admixture of brown-tipped hairs; plumbeous under- 

 fur exposed in many areas of dorsum; rostral, pectoral, cheek, flank, 

 and mystacial areas, and entire venter Pale Ochraceous-Buff; lateral 

 areas washed with darker color; fore and hind feet sparsely haired 

 and white dorsally; ventral surface of hind feet with long tufts of gray 

 hairs; tail indistinctly bicolored, Cinnamon-Buff dorsally, Light Buff 

 ventrally and with a prominent pinna, black proximally and white 

 distally. Skull; Small, zygomata delicate, auditory bullae small and 

 little inflated, rostrum narrow, foramen magnum small, molariform 

 toothrow short, and angular process of mandible with two distinct 

 foramina on its anteromedial surface. 



Remarks. This subspecies differs from all other subspecies of 

 Jaculus deserti in the smaller size of all cranial and external measure- 

 ments. The type specimen of J. d. varus resembles the type series of 

 Jaculus jaculus cufrensis in body size, greatest length of skull, and 

 condyloincisive length. In crown length of upper molariform toothrow, 

 least interorbital breadth, length of nasals, breadth of rostrum at 

 level of antorbital foramina, size and degree of inflation of auditory 

 bullae, and in external measurements, it is closer to Jaculus jaculus 

 butleri Thomas. The above are only superficial resemblances, however, 

 inasmuch as this specimen has two foramina in the angular process of 

 the mandible, brownish hairs on the soles of the hind feet, and markedly 

 darker dorsal coloration with greater suffusion of brownish hairs on 

 the rump and sides. The latter are all diagnostic characteristics of 

 Jaculus deserti and, in the aggregate, are unknown in Jaculus jaculus. 



This single specimen was obtained from the barren hamada at the 

 base of the Gebel near Ain Zueia by Henry W. Setzer, while en route 

 to the Tibesti Mountains in the spring and early summer of 1961. 

 It was obtained only after three successive nights of continuous trap- 

 ping throughout an area of abundant diggings. Efforts to obtain ad- 

 ditional specimens by "night hunting" were without effect. 



The extensive sand seas to the north have apparently isolated this 

 subspecies from the ranges of all other populations of Jaculus deserti. 

 The small size and other cranial differences of this subspecies may 

 have arisen through the agency of genetic drift and consequent rapid 

 genetic fixation in an effectively small, isolated population of jerboas. 



The term rams is from the Latin meaning scarce or scattered and 

 has reference to the apparent scarcity of jerboas of this subspecies. 



