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eminently Sonoran, and its arborescent fornis, three in number (gigan- 
teus, thurberi, and schottii), are strictly confined to the desert region of 
the Lower Colorado, chiefly upon the Arizona side in the region of the 
Gila River; but they are merely the northern representatives of the 
large display of giant forms found in Sonora and Lower California. 
Of the nonarborescent forms of EUCEREUS, but two are included in 
our flora, one (emoryi) being a Lower Californian type extending into 
southern California, and the other (greggii) a Sonoran and Chihuahuan 
type extending into Arizona and as far east as the Pecos River in 
Texas, by far the most eastern of our EUCEREI. It will be seen that 
there are no species of EUCEREUS peculiar to the United States. 
The relatively larger display of the ECHINOCEREI in the United 
States is probably to be partially explained by their low compact forms, 
simulating in a perplexing way those of Cactus and Eehinocactus. Out 
of the twenty-three species of our flora but two are really cylindrical, 
one ( poselgeri) a form occurring on both sides of the Lower Rio Grande 
and so weak and slender that it uses shrubs for a support; the other 
(berlandieri) also a form of southeastern Texas, originally found on the 
Nueces, apparently peculiar to the United States, and with its short 
cylindrical body forming a transition between the cylindrical and more 
compact forms. The remaining twenty-one species form our dominant 
Cereus flora, and of these but nine are peculiar, so far as now known, 
to the United States. These twenty-one species are easily thrown into 
three groups: (1) the pectinate forms; (2) the purple-flowered nonpec- 
tinate forms; and (3) the scarlet-flowered nonpectinate forms. The 
first contains seven species, the second six species, the third eight 
species. 
The pectinate forms are characteristically eastern in their display 
and have the greatest northern extension, viridiflorus reaching the 
mountains of southern Wyoming. Their Mexican origin is also more 
in the direction of Coahuila and Chihuahua than of Sonora. Of the 
seven species represented, three are peculiar to the United States 
(viridiflorus, chloranthus, and dasyacanthus), Of these three forms 
only viridiflorus has a northern range, extending from the mountains 
of southern Wyoming through Colorado to the borders of eastern New 
Mexico and northwestern Texas, but it is represented in the El Paso 
region by its cylindrical variety, tubulosus. In the case of the two 
other species, chloranthus has a very restricted range, being confined to 
the Kl Paso region, while dasyacanthus, occurring in the same region, 
extends westward to Arizona. It is more than probable that both of 
these species will be found in Mexico. The closely associated ctenoides 
is also a form of southwestern Texas, which has come from Coahuila 
and Chihuahua. Another species of Chihuahuan origin, oceurring in 
the El Paso region but extending westward to Arizona, is roetteri. 
The strongest pectinate type, however, is exhibited by the species 
pectinatus, which is common to Chihuahua and Sonora, but is repre- 
