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of the Tucson Valley in the Santa Catalina Mountains).
It is very likely that they occur in the remaining
mountain ranges south of the Gila River,
although the Baboquivari Mountains may be inhabited
by C. collaris (Axtell and Montanucci, 1977).
Crotaphytus bicinctores generally skins the northern
border of C. nebrius' range on the north side of the
Gila River, but crosses the river at the Sentinel Plain.
a region uninhabited by C. nebrius, as no Sonoran
mountain ranges project northward into this area.
In Sonora, Cro~aphyfus nebritls occurs in the
transversely oriented foothills that follow the United
States-Mexico border along Mexican Highway 2
(the Pinacate Region). The north-south trending
ranges of southwestern Arizona project northward
from these foothills and probably provide the corridor
through which C. nebritcs entered these mountains.
They have been collected from several mountain
ranges to the south and east in northern Sonora
including the Sierra Cubabi, Sierra La Gloria, Sierra
El Alamo, and Sierra El Rajon. One specimen is
known from either the Sierra Cibuta or Sierra El
Pinto (AMNH 73758, 25.6 km S Nogales), a more
eastern locality in the northern foothills of the Sierra
Madre Occidental. There is a relatively large gap in
the known distribution of the species between the
Caborca region (Sierra El Rajon) and the Hermosillo
region. However, a series of specimens are known
from the foothills between Hermosillo and the
Guaymas region. Finally, the remaining specimens
have been taken from the foothills of the Sierra
Madre Occidental, in a series of north-south trending
valleys separated by presumably uninhabitable
densely vegetated mountain ranges. It is likely that
C. nebrius reached these localities by way of major
river drainages entering from the south, such as the
Rio Sonora and Rio Yaqui, as suitable open habitat
appears to be restricted to these drainage systems.
Populations of Crofaphytus nebrius are only narrowly
separated from those of C. bicincrores at two
localities and in both cases the bamer that prevents
contact is the Gila River. Crotaphytus nebrius occurs
on the northern edge of the Gila Mountains and is
separated from a population of C. bicincrores in the
Laguna Mountains approximately 0.4 km to the
north on the opposite side of the Gila River. Similarly,
C. nebrius occurs on the western margin of
the Buckeye Hills, while C. bicinctores occurs on the
extreme eastern margin of the Gila Bend Mountains
only a few hundred meters to the west on the opposite
shore of the Gila River. Thus, C. nebrius may
be observed on the east side of the Gillespie Bridge
OF CROTAPHYTID LIZARDS
and C. bicinclores can be observed moments later
on the west side.
Several questions remain regarding the distribution
of C. nebrius. First, C. nebrius occurs as far
nonh as 1 1.7 km N Huasabas and 19.5 km N Bacadehuachi
in the Sierra Madre Occidental, while
C. collaris is known from as far south as the Bavispe
Region, approximately 60 km to the nonh. It is
unknown whether this gap is real or an artifact of
collecting. The habitat in the Huasabas and Bacadehuachi
regions appears to be marginal and the
presence of higher elevation mountains between this
area and the Bavispe region strongly suggests that a
contact zone does not exist here. However, this remains
to be substantiated with additional field studies.
Second, a specimen of C. bicincrores was observed
by the author at Black Gap, Maricopa County,
Arizona, a narrow pass on the western periphery
of the Sauceda Mountains through which Arizona
State Highway 85 passes. This observation was extremely
surprising given that this area is apparently
well isolated from known C. bicincrores populations
north of the Gila Bend River and on the Sentinel
Plain. If C. bicinctores has an established population
at this locality, it is likely that C. nebrius and C.
bicinctores contact somewhere in the Sauceda or
Maricopa mountains. Several later attempts to find
C. bicincrores or C. nebrius at this locality were unsuccessful.
Fossil Record. -Van Devender and Mead ( 1 978)
referred a maxilla and dentary from late Pleistocene
deposits in the Tucson Mountains and Wolcott Peak,
Pima County, Arizona, to Crotaphy~us collaris. Van
Devender et al. (1991) referred dentary, maxillae,
and tooth crown material from late Pleistocene deposits
in Organ Pipe Cactus National Monument to
either C. collaris or C. insularis. Because the Tucson
Mountains and Organ Pipe Cactus National Monument
are currently inhabited by C. nebrius, this
material probably should be referred to C. nebrius
on distributional grounds.
NatltralHislory. -Nothing has been published regarding
the natural history of this species but I have
made the following observations. Croraphj)rics nebrius
occurs in a diversity of habitats, although always
in association with rocks. In the northern portion
of its range it may be found in extremely xeric
habitats characterized by granitic outcroppings or
volcanic flows. In the southern portion of its range,
it is occurs in rocky areas often with relatively dense
arid-tropical thornscrub vegetation. In these areas,
C. nebritcs may be concentrated in arroyo bottoms