It is only possible to deal with the forms that occur within the borders of the United States, as even individual stations of common Mexican forms are little if at all known.These United States forms represent a northern extension of an abundant Mexican display.The group EUMAMILLARIA, containing twelve of the thirty-one forms defined as occurring north of the Rio Grande, makes the feeblest extension northward,at no place being found far from the boundary, and all the twelve are Mexican forms which extend but slightly into the United States.Only five of the forms are found east of the Pecos: heyderi, the most widely distributed EUMAMILLARIA, extending from the southeastern border of Texas westward along the whole Mexican boundary except in California; hemisphaericus, extending through southern Texas and southern New Mexico; meiacanthus, also along the Mexican border of Texas and New Mexico; texanus, a low ground form of the Rio Grande Valley, extending from the mouth of the river to El Paso, and suggesting a connection with the West Indian stellatus; and sphaericus, another low ground valley form of similar range, but apparently only extending up the Rio Grande to the region of Eagle Pass.
The Pecos forms the eastern boundary of five other EUMAMILLARIA forms: micromeris, extending northward from Coahuila and Chihuahua, apparently only in the mountains between the Pecos and El Paso; wrightii, of similar narrow northward extension, but ranging further northward on the high plains of the Upper Pecos in New Mexico; denudatus, also with a narrow northward extension west of the Pecos; lasiacanthus, extending from Chihuahua with a northern limit between the Pecos and Arizona; and grahami, a Sonoran type which has spread between the Pecos and southeastern California.
The ten preceding forms have evidently entered our borders from the highlands of Sonora and Chihuahua, with the exception of the Rio Grande Valley forms, texanus and sphaericus.Another species, tetrancistrus, is also a Sonoran type which has reached the eastern slopes of the mountains of southeastern California, and extended through western Arizona to southern Nevada and southern Utah, the most extended northern range of any EUMAMILLARIA.The twelfth form, goodrichii, is Lower Californian, and extends into California only in San Diego County.A summarized statement of the distribution of our twelve EUMAMILLARIA would be that two of them have extended from the low grounds of Coahuila and Chihuahua and spread along the valley of the Rio Grande; nine have come from the high grounds of Chihuahua and Sonora, four of which have extended eastward to the low levels of southeastern Texas; four have keptto the highlands west of the Pecos, and one has kept to the Colorado Valley and its tributaries, while one has a short northern extension from Lower California.
The nineteen forms of CORYPHANTHA are decidedly more northern in their distribution, and are our characteristic representatives of the genus Cactus.Ten of these, however, are but northern extensions of Mexican forms, and six of the ten have simply that tongue-like northern extension in the mountains between the Pecos and the Upper Rio Grande (above. El Paso), viz.: dasyacanthus, tuberculosus, scheerii (which has also spread somewhat east of the Pecos), and the three pectinate and closely related forms radians, echinus, and scolymoides.Of the four remaining Mexican forms, macromeris is a low ground Rio Grande Valley form, extending from above El Paso well towards the Lower Rio Grande; potsii just crosses the border in the neighborhood of Laredo; and radiosus and neo- mexicanus have by far the greatest northern extension, stretching from Sonora and Chihuahua to southern Utah and central Colorado, and eastward to the Guadalupe River of Texas.
The nine remaining coryphanths are distinctly forms of the United States, occupying two well-marked regions, viz.: the northern plains, and the desert region of western Arizona and adjacent California, Nevada, and Utah.In the former region is found the widespread viviparus, which extends from the southern borders of British America to the plains of eastern Colorado and western Kansas, and even crosses the Rocky Mountain divide into northern Idaho and northeastern Washington; and missouriensis, which also ranges from the high prairies of the Upper Missouri to the same southern limit, and is continued southward into Texas in its varieties similis and robustior.
In the Arizona desert region, four distinct but closely allied forms have become differentiated from the strong radiosus stock, viz.: arizonicus, deserti, alversoni, and chloranthus, all of which might be regarded as distinct species.In southeastern Texas is found an isolated form, sulcatus, occurring between the Brazos and Nueces rivers.That viviparus must be regarded as a strong northern extension of the radiosus stock can not be doubted, as the low depressed cespitose northern form seems to mergesouthward so gradually into the simple more robust ovate to cylindrical forms of radiosus as to suggest the propriety of regarding them all as specifically identical.
