124 PROCEEDINGS OF THE ACADEMY OF [March, 
‘f.. . the community in reptiles between the Sonoran and the 
Austroriparian is much less than that between the Sonoran and the 
Central; it is, in fact, limited to widely ranging genera .... the 
separate identity of these two faunas [Sonoran and Austroriparian], 
as far back as they can be traced, confirms the conclusion already 
reached, from their present distribution, that the association of the 
Sonoran with the Austroriparian into one transcontinental zone is 
unnatural.’’ 
This terse statement only requires the substitution of ‘‘mollusks’’ 
for ‘“reptiles’’ to serve as a summary of the results of our present in- 
vestigation. In land mollusks, as in reptiles, the Sonoran types have 
pushed farther into the humid Austroriparian than the eastern types 
into the Sonoran area. Bulimulus and the texasiana group of Poly- 
gyra are cases in point; while Eastern subgenera of Polygyra, and prac- 
tically all Austroriparian forms except the minute, widely ranging 
genera, stop short at the rise marking the approximate limit of the 
Cretaceous formation and the Sonoran area in Texas. The common 
boundary of the Austroriparian and Sonoran, while surprisingly sharp 
for continuous areas, is deeply accidented by the river valleys, which 
carry very narrow Austroriparian ramifications into Sonoran borders. 
Thus, along the San Marcos river in Hays county, the Guadalupe 
river in Comal county, and the San Antonio river in Bexar county, 
an Austroriparian fauna with such Eastern forms as Omphalina, Poly- 
gyra thyroides, P. monodon fraterna, P. auriformis, P. roemeri, Bulimu- 
lus d. liquabilis, ete., is found, while the fauna on the bluffs or away 
from the streams is frankly Sonoran. Farther southwest, the Aus- 
troriparian forms are wholly wanting, even where moist local condi- 
tions prevail, as in the immediate vicinity of the streams of Val Verde 
county. 
A list of the forms characteristic of the eastern or lower portion of 
the Sonoran in Texas (Texan District of Cope) follows: 
Praticolella berlandiervana Strobilops |. texasvanus 
Thysanophora hormi Zonitoides nummus 
Polygyra hippocrepis Vitrea indentata umbilicata 
Polygyra texasiana hyperolva Vitrea dalliana roemeri 
Polygyra t. texasensis Euconulus chersinus trochulus 
Polygyra mooreana’ Helicodiscus eigenmanni 
Bulimulus d. mooreanus Planorbis carus 
Bulimulus d. ragsdaler Planorbula obstructa 
5 This species extends somewhat into the Austroriparian. 
or 
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