544 



MOLLUSCA 



PHYLUM VI 



Family 8. Valvatidae Gray. 



Shell composcd of few ^vhorls, conical or discoidal, umhilicate. Ajjerture round, with 

 continuous 'peristome. Opercuhim horny, circular, multisjjiral. 

 Upper Jura to Kecent. 



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



froiu turbinate to discoidal. It comprises about twenty-five 



Fio. 950. Eecent species, inliabiting tlie fresh waters of Eiirope and 



Valvata piscinaiis Müll. Nortli America, It is initiated in tlie Purbeck, biit does not 



?ransVivanr'''' ^^'^^'' become at all abundant until tlie Tertiary. 



Family 9, Viviparidae Gill. 



Shell conical or ttirhinate, with thick epidermis ; imjyerforate or with narrow 

 umhilicus. Whorls smooth, tubulär or angular. Äpertiire rounded, oval, suh-angular 

 posteriorly, with continuous peristome. Operculum horny, concentrically striated, with 

 eccentric nucleus. Jura to Eecent. 



Tliis, tlie principal genus, is 

 G D 



Vivipara Montf. [Paludina Lam.) (Fig. 951). 

 abundant in fresh water a b 



of all parts of the globe, 

 with the exception of 

 tropical and South 

 America. Several other 

 genera and subgenera 

 are . recognised, such as 

 Gampeloma Kaf. (Me- 

 lantho auct.) of North 

 America, comprising 

 mostly smooth, thick- 

 shelled species, with 

 thickened inner lip ; 

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

 Nev., China ; Lioplax Troschel ; Laguncula Benson ; Tylopoma, Boshovicia 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, Eouniaiiia 

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

 Neiimayr has described a number of mutation series from this horizon, wliich begin 

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



Fig. 951. 



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

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

 Slavonia. 



Family 10. Hydrobiidae Fischer {Ämnicolidae Tryon). 



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

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

 concentric. Cretaceous to Eecent. 



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. 



Bithinia Gray (Fig. 952). Thin-shelled, turbinate, with umbilical fissure. 

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

 to Eecent. 



