VALLONIA. 285 



This very distinct genus of minute snails occupies the entire 

 Nearctic and Palsearctic regions, and some species have been intro- 

 duced (probably with plants) into Australia, Mauritius, etc. They 

 are gregarious in habit, and live under fragments of wood, stones, 

 on mossy cliffs and in damp meadows, always avoiding light. The 

 number of species is very uncertain ; but whether species or varie- 

 ties, there can be no doubt that a considerable number of recognizable 

 forms must be distinguished. The arrangement given below is that 

 of Dr. Sterki, who has made special studies on a far greater mass of 

 material than any other observer. An alternative to this classifica- 

 tion would be to make excentrica a variety of pulchella, and unite 

 adela, declivis and pollinensis under the former of the three names ; 

 all the forms of the costata group might then fall under eostata as 

 varieties. Having seen neither mionecton, ladacensis nor asiatica I 

 do not care to suggest any mode of uniting the forms included in 

 Sterki's " Group of F cyclophorella." 



Group of V. pulchella. 



V. pulchella Mull., viii, 248. v. enniensis Gredl. 



rosalia Risso (part). v. hispanica Sterki. 



paludosa Da Costa. v. persica Rosen. 



erystallina Dillw. V. excentrica Sterki, viii, 249. 



Icevigata Moq. V. adela West., viii, 251. 



nitidula Stud. V. declivis Sterki, viii, 251. 

 potua Chier. v. altilis Sterki. 



minuta Say. V. pollinensis Paul., viii, 252. 



Group of F costata. 



V. costata Mull., viii, 252. V. albula Sterki. 



alexandrce Cox. V. parvula Sterki, viii, 254. 



rosalia Risso (pt.). V. tenera Reinh., viii, 255. 



crenella Mont. pukhellula Hde. 



helicinus Lightf. v. patens Reinh., viii, 257. 



v. helvetica Sterki. V. gracilicosta Reinh., viii, 256. 

 v. amurensis Sterki. 

 v. pyrenaica Sterki. 

 v. montana Sterki. 



