86 
TRANSACTIONS OF THE TEXAS ACADEMY OF SCIENCE. 
enough to enable us to classify even the former with the neotropical re¬ 
gion. The Lower Rio Grande valley is about one-fourth Tropical in the 
general aspect of its insect fauna, so far as m_y**present data go. This 
includes also a maritime Antillean insect fauna on Padre Island, indi¬ 
cated by Macrancylus sp., Oxacis spp., etc. 
ADDITIONAL REMARKS. 
Upper and Lower Sonoran. —The generally accepted idea of the 
Lower Sonoran is that it comprises all arid territory in which the sub¬ 
tropical fruits flourish. This point is of much importance from an econ¬ 
omic standpoint. The date palm is most characteristic of subtropical 
arid regions the world over. It is thus particularly valuable in defining 
the northern limits of the Loioer Sonoran. 
In the Lower Sonoran , light frosts often occur, but heavy frosts and 
snowfalls are extremely rare and very exceptional to the general rule. 
In the Upper Sonoran killing frosts and snowfalls are usual, the frosts 
alone in the exceptional absence of snow being sufficiently severe to kill 
subtropical fruits if left exposed. 
In the Lower Sonoran the mesquite (P. juliflora) assumes tree-like 
proportions, while yuccas become arborescent, and in the warmest and 
dryest portions cacti attain their greatest development. The pitahaya 
cactus is an example in point. It grows in Sonora and Arizona, as well 
as in the region of Oaxaca. The arborescent tuna cactus obtains more in 
the north of the tableland proper. Arborescent yuccas occur in the 
latter region also, and Y. arborescens is found in the highest parts of the 
Lower Sonoran on the west side. 
In North American Fauna No. 7 (Part II., Report on Death Valley 
Expedition, 1893), p. 286, Dr. Merriam says: “The creosote bush 
(Larrea tridentata) is the most conspicuous, most widely distributed, and 
best known bush of the torrid deserts of the southwest, where it covers 
the gravel soil up to a certain line, which probably marks the southern 
limit of killing frost. The Larrea belt is the most important of all from 
the horticultural standpoint, because it is suited to the requirements of 
the citrus fruits, the olive, almond, fig, and raisin grape.” I have rea¬ 
son to believe that this is very true of the Californian and western Ari¬ 
zona region, but its application wholly ceases when applied to the Atlan¬ 
tic slope, or the region east of the Continental Divide, in New Mexico, 
Chihuahua, and Texas. In New Mexico, the Larrea withstands both 
snows and killing frosts every winter; and in Chihuahua and Texas, 
killing frosts at least, if not always snows. So far as my observation 
goes, Larrea never descends into the Lower Sonoran on the Atlantic 
slope. 
