544 



MOLLUSCA 



PHYLUM VI 



Family 8. Valvatidae Gray. 



Shell composed of few ivJiorls, conical or discoidal, umhilicate. Aperture round, tvith 

 continuous ijeristome. Operculum horny, circular, multispiral. 

 Upper Jura to Recent. 



The genus Valvata MiiU. (Fig. 950) is small, and varies 



from turbinate to discoidal. It comprises about twenty-five 



Pig. 950. Eecent species, inhabiting the frcsli waters of Eiiroj^e and 



Valvata piscinaiu Miiii. North America. It is initiated in the Purbeck, but does not 



Transylvauia.*^'^'' ' ^'^°^*'' become at all abundant until the Tertiary. 



Family 9. Viviparidae Gill. 



Shell conical or turbinate, tvith thick epidermis; imperforate or tvith narroiv 

 umbilicus. Whorls smooth, tubular or angular. Aperture rounded, oval, sub-angular 

 posteriorly, with continuous peristom.e. Operculum horny, concentrically striated, ivitli 

 eccentric nucleus. Jura to Eecent. 



Vivipctra Montf. {Paludina Lam.) (Fig. 951). 



B 



This, the principal genus, is 

 a D 



abundant in fresh water 

 of all parts of the globe, 

 with the exception of 

 tropical and South 

 America. Several other 

 genera and subgenera 

 are recognised, such as 

 Campeloma Raf. {Me- 

 lantho auct.) of North 

 America, comprising 

 mostly smooth, thick- 

 shelled sj^ecies, 

 thickened inner lip ; 

 Tulotoma Haldem., including forms with angular whorls. North America ; Margarya 

 Nev., China ; Lioplax Troschel ; Laguncula Benson ; Tylopoma, Boslwvicia Brusina, etc. 

 Typical species of this genus are found in the Wealden clays. Vast numbers of 

 Vivipara occur in the Pliocene of southern Hungary, Croatia, Slavonia, Roumania 

 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 Vivipara species, and terminate with angular Tulotoma-\ik.& forms. 



Fig. 951. 



A, B, Vivipara hrusinae Neumayr. C, V. (Tulotoma) forbesi Neumayr. 

 with Pliocene ; Isle of Cos. D, V. {Tulotoma) hoerneri Neumayr. Pliocene : Novska, 

 Slavonia. 



Family 10. Hydrobiidae Fischer {Amnicolidae Try on). 



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

 ribbed or spirally heeled. Aperture ovate; opierculum horny or calcareous, spiral or 

 concentric. Cretaceous to Recent. 



Tliese 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. 



Bithinia Gray (Fig. 952). Thin-shelled, tui'binate, with umbilical fissure. 

 Peristome continuous, outer lijj sharp. Operculum calcareous, concentric. Wealden 

 to Recent. 



