Paludestrina protea (Gould) 

 Valvata tricarinata (Say) 



With the exception of the Glabaris all occur in the Ari- 

 zona System. 



£ 



■4-> 



Rio Grande System. & | 



2 5 



s* c 



.2 T 



PS < 



(Elliptio?) popei (Lea) K 



Corneocyclas compressa (Prime) 



Corneocyclas pulchella (Jenys) 



Lymnaea columella (Say) 



Lymnaea solida cubensis (Pfeiffer) 



Lymnaea truncatula (Miiller) 



Gundlachia rivularis (Say) 



Planorbis antrosus (Conrad) 



Planorbis trivolvis (Say) 



Segmentina parva ( Say ) ' 



Segmentina liebmanni (Dunker I 



Segmentina umbilacatellus (Coekerell) 



Segmentina alba (Miiller) 



Segmentina exacuoa ( Say ) 



Paludestrina protea (Gould I 



Valvata tricarinata ( Say ) 



The fauna of this region presents no particular affinities 

 with that of the Arizona System; only the more widespread 

 species constitute a common element. 



Factors of Distribution. 



Jordan defines distribution by saying that every kind of 

 animal is found in all parts of the world except where it can- 

 not get, where it has gotten but cannot maintain itself, and 

 where it lias gotten but evolved into something else. Dispersal 

 of land animals is the resultant of two factors, life-zones or 

 belts of equal temperature and humidity which permit free lat- 

 eral migration, and geographic barriers such as seas to non- 

 maritime, prairies to arboreal, and deserts to amphibious 

 genera which divide these zones into faunal areas. Water 

 owes its temperature to many influences of which that of the 

 adjacent land-surface is but a factor. An aquatic animal, such 

 as a fish or mollusk, once in a river system, unless the thermal 

 conditions are absolutely prohibitive, will eventually spread 

 over the entire area and multiply at favorable localities re- 

 gardless of life-zones since its migration is proportionally 

 many times more rapid and not impeded by absence of food 

 and shelter. As a result one of the chief aids in the classifica- 



41) 



