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. Tryon. 
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 land. 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. 
A C 

Fic. 903. 
A, Bythinia tentaculata, { aa 
Linn. sp. Upper Mio- Fic. 905 Fic. 906. 
cene ; Miocic, Dalmatia. Fic. 904 neg AUR Hydrobia (Litori- 
B, Opereculum of same. = F Nystia Chastelii, nella) acuta, A. 
C, B. gracilis, Sandb. Nematura pupa, Nyst. Nyst. sp. Middle Braun. Miocene ; 
Fresh-water Molasse ; sp. Oligocene ; Hacken- Oligocene; Klein- Weissenau, near 
Oberkirchberg, near Ulin. heim, near Alzey. Spouwen, Belgium. Mayence. 
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 ; Towrnoueria, 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, Prév. sp. A, Pyrgula Wugentae, Noetee ee 
Other genera and sub-genera closely related to Miocene; Arpatak, Transylvania. — B, 
the foregoing are Bythinella, Moq. ; Amnicola, Gould ; BO. epee casas aiteen nS 
Belgrandia, Lartetia, Bourguignat; Lapparentia, C, Mohrensternia inflata, Andrzewsky. 
; 3 Congerien Stage (Miocene); Inzersdorf, 
serthelin. near Vienna. } 
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); Mohrensternia, 
Stol. (Fig. 907, C); Pyrgidium, Tournouer ; Prososthenia, Neumayr. Tertiary. 
A B Cc 

Fic. 907. 
