124 PROCEEDINGS OF THE ACADEMY OF [March, 



". . . . 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 oiu- 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, Bidimu- 

 lits d. liquahilis, etc., 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 inunediate 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 berlandieriana Strohilops I. texasianus 



Thysanophora hornii Zonitoides nummus 



Polygyra hippocrepis Vitrea indentata umbilicata 



Polygyra texasiana hyperolia Vitrea dalliana roemeri 



Polygyra t. texasensis Euconulus cher sinus trochulus 



Polygyra mooreana^ Helicodiscus eigenmanni 



Bulimulus d. mooreanus Planorhis carus 



Bulimulus d. ragsdalei Planorbula obstructa 



* This species extends somewhat into the Austroriparian. 



