120 U.S. NATIONAL MUSEUM BULLETIN 275 



color, are indistinguishable from gerbils from Socna and Bir Fergian. 

 Members of this subspecies from various localities in the coastal plain 

 tend to have slightly larger, more bulbous auditory bullae. Those 

 from 12 kilometers west of Sirte are also darker in dorsal color. This 

 variation is well within the expected limits of a subspecies whose 

 range includes such a large geographic area. 



The populations from Socna and Bir Fergian show only slight 

 intergradation with G. p. tarabuli whose range is to the south. Gerbils 

 from the type locality at Derg show no evidences of interbreeding 

 with populations to the south. The high escarpments of the Hamada 

 de Tinrhert in the west and the mountain ramparts of the Gebel es 

 Soda and the Gebel el Harug el Asued in the east obviously have 

 proven to be serious deterrents to gene exchange between populations 

 of these gerbils, and thus the range of this subspecies is limited to 

 the hamadas, coastal deserts, and coastal plain lying north of these 

 barriers. In northwestern Tripolitania the range of this subspecies ap- 

 parently does not attain the Mediterranean Coast. Three specimens 

 from 5 kilometers north of Mizda constitute the northernmost record of 

 occurrence of G. pyramidum in Libya. Farther north, Gerbillus aureus 

 is the dominant gerbil and probably competes with and thus prevents 

 G. pyramidum from occupying the coastal areas. Farther eastward, 

 however, along the Gulf of Sirte, G. aureus is known only from Sirte, 

 while G. p. hamadensis is known from several localities. 



Ecological observations. The type series was obtained from the 

 rather denuded margins of a small wadi in the western portion of 

 the Hamada el Hamra. This dry watercourse afforded scanty vege- 

 tative cover, while rocky outcroppings interspersed with bare areas 

 of sand and gravel were widespread. In the immediate vicinity of the 

 wadi, Tamarix, Calligonum, and other types of shrubs were the dom- 

 inant vegetation, but farther out on the hamada, smaller plants 

 growing close together were commoner. The soil in this area consisted 

 mostly of sand. 



Setzer (1957) describes the habitat at Socna and Bir Fergian as 

 consisting of a sandy-clay substrate similar to hardpan, which re- 

 sembles the desert playas of the Western United States. The collecting 

 sites in the coastal plain near Sirte, Marble Arch, and El Agheila 

 were characterized by dense plant cover and the soil in these areas 

 also was sandy-clay. Large, permanent dunes supporting Calligonum 

 characterized the habitat near Bu Ngem. Judging from the sandy 

 character of these habitats, this subspecies always requires a sandy 

 substrate. 



The name hamadensis refers to the hamada-like character of the 

 habitat at the type locality and several other collecting sites of this 

 subspecies. 



