464 



MOLLUSCA 



SUB-KINGDOM VI 



and the Island of Cos, where they are remarkable for their extreme variability. 

 Neumayr has described a number of mutation series from this horizon, which begin 

 with smooth Viviparus species, and terminate with angular Tulotoma-like forms. 



Family 10. Amnicolidae. Try on. 



Shell turbinate to turreted, small, usually thin, and either smooth, longitudinally 

 ribbed, or spirally keeled. Aperture ovate; operculum horny or calcareous, spiral or 

 concentric. Cretaceous to Recent. 



These are fresh or brackish water inhabitants, some of which, however, are able to 

 survive for a considerable period on lajid. It is difficult to distinguish the different 

 genera belonging to this family by means of shell characters alone. 

 All the forms are diminutive. 



Bythinia, Leach (Fig. 903). Thin -shelled, turbinate, with 

 umbilical fissure. Peristome continuous, outer lip sharp. Oper- 

 culum calcareous, concentric. Wealden to Recent. 



Staliola, Brusina. Outer lip thickened ; operculum calcareous. Cretaceous to 

 Miocene. 



Fossarulus, Neumayr. Like the last, but with spiral ribs. Upper Miocene. 



Nematura, Benson (Stenothyra, Benson), (Fig. 904). Like Bythinia, but aperture 

 contracted. Operculum calcareous, spiral. Tertiary and Recent. 



Nystia, Tourn. (Forbesia, Nyst.), (Fig. 905). Outer lip reflected ; operculum 

 calcareous, spiral. Tertiary and Recent. 



Assiminea, Leach. Tertiary and Recent. 



Hydrobia, Hartm. (Littorinella, Braun ; Tournoueria, Brusina), (Fig. 906). Conical 

 to turreted, acuminate, smooth. Aperture oval ; operculum horny, paucispiral. 

 Cretaceous to Recent. The Indusia Limestone (Lower 

 Miocene) of Auvergne is almost exclusively composed 

 of the shells of H. Dubuissoni, Bouill. Similarly, the 

 Littorinella Limestone of the Main Basin which is of 

 equivalent age, is made up of the shells of H. acuta, 

 Braun. Strata in the fresh-water limestone of Nord- 

 lingen are charged with H. trochulus, Sandb. ; and 

 the Upper Eocene marl of St. Ouen is filled with 

 the remains of H. pusilla, Prev. sp. 



A,Pyrqula Eugeniae, Neumayr. Upper 



Other genera and sub -genera closely related to Miocene; Arpatak, Transylvania. B, 



Onnlr] Micromelania(Diana) Haueri, Neumayr. 

 ^ 1Q , gp upper Miocene; Miocic, Dalmatia. 



Lapparentia, C, Mohrenstemia inflata, Andrzewsky. 

 Congerien Stage (Miocene) ; Inzersdorf, 

 near Vienna. 



FIG. 907. 



thp fnrprrrvmo- arp Riit'h'itiplln Mnn 

 Ua, MOq. , 



Belgrandia, Lartetia, Bourguignat ; 

 T, .-, , 



lin - 



Pyrgula, Christofori and Jan. (Fig. 907, A}. 



Turreted, whorls spirally keeled or ribbed. Peristome continuous. Tertiary and 

 Recent. 



Genera allied to the last are Micromelania, Brus. (Fig. 907, B) ; Mohrenstemia, 

 Stol. (Fig. 907, C) ; Pyrgidium, Tournouer ; Prososthenia, Neumayr. Tertiary. 



