260 RISSOID^, 



united with the tentacles, but it seems that their length increases, 

 the more the animal is accustomed to an amphibious life. Thus 

 some of the species have the eyes placed near the basis of the 

 tentacles, others in the middle, and still others at the tips of the 

 same. The differences are very gradual, which makes it impos- 

 sible to regard them as of any important generic value. The 

 length of the rostrum also appears to increase in some forms, 

 corresponding with their more amphibious habits ; and also, 

 while the foot itself often lengthens, its disk at the same time 

 becomes smaller. 



Subfamily BYTHININM. 



Shell small, conical ; peritreme simple or thickened. Oper- 

 culum calcareous, concentric. Fresh water. 



Bythinia, Gray. 



Syn. — Elona, Moquin-Tandon. Grayana, Betta. 



Distr. — 50 sp. Europe, Southern Asia, Australia, etc. Fossil. 

 Wealden and Tertiary. B. Leachii, Shepp. (Ixxii, 29). 



Shell oval, turbinated, thin, invested with a thin epidermis; 

 peritreme continuous. 



The female is oviparous and deposits her eggs in a band, 

 attached to stones or the stems of aquatic plants ; with her mouth 

 she clears the surface upon which she intends to deposit the ova. 

 The young are hatched in three or four weeks, attaining full 

 growth in the second yea,Y. 



TYLOPOMA, Brusina, 1882. Shell form of Tulotoma (Palu- 

 dinldse ), but smaller. Operculum calcareous. B. avellana, ISTeum. 



GABBiA, Tryon. Shell turbinately globose, whorls well rounded ; 

 peritreme continuous ; axis perforate. Operculum, nucleus sub- 

 spiral, afterwards concentric, calcareous. Distr. — B. australis^ 

 Tryon (Ixxi, 23). Fresh water, Australia. It is possibly synon- 

 ymous with Bythinia, from which it only differs slightly in its 

 operculum. 



Stenothyra, Benson. 



Syn. — Nematura, Benson. 



Distr. — Fresh water, India, East Indies. Fossil. Eocene ; 

 Paris basin. S. deltae, Benson (Ixxii, 31). 



Shell ovate, smooth, imperforate; aperture rounded, contracted. 

 Operculum testaceous, annular, ovate, thick, nucleus subcentral, 

 margin grooved. 



The species are found either attached to the under surface of 

 floating leaves, or crawling out of the water on the muddy 

 margins of ponds, leaving, as they progress, slender tracks 

 behind them. 



NEMATURELLA, Saudbergcr. Like Stenothyra, but with longer 

 spire ; lip flexuous, forming a slight sinus above. Operculum 

 unknown. Pliocene ; Europe. 4 sp. 



