34 YALVATIM;. 



swimming. Shell conical, thin, covered with 

 an olive periostraca ; mouth round, with a con- 

 tinued simple peristome. Operculum horny, 

 suborbicular, formed of many gradually enlarg- 

 ing whorls, which have a raised membranaceous 

 outer edge, forming a spiral ridge on the outer 

 surface (f. 9. .). 



The shells are known from Paludinae by the shelly 

 cone being circular, and not bent in in any part by 

 the proximate whorls. 



1. VALVATA Mutter. (Valve Shell.) 



Shell with the spire a little elevated, or flat and disk- 

 like ; aperture quite circular, united all round, 

 and furnished with a horny operculum marked 

 with a single raised spiral membranaceous line. 



So named from the valve or lid which covers the 

 orifice of the aperture. 



7. 1. VALVATA pisdnalis. Stream Valve-shell. 

 Shell globular, with an elevated obtuse spire, 

 and a deep central umbilicus, (t. 10. f. 1 14.). 



The lingual plate of V. pisdnalis is elongate; 

 Fig. 7. the central tooth sub- 



quadrate, base pro- 

 duced on both sides, 

 incurved, cutting edge 

 b/ ^ ^. -, triangular, the middle 



Teeth of Valvata. N^ with ft pec tinated 



, central; 6, c,d, lateral. t * . ., . 



lobe on each side ; late- 

 ral teeth somewhat similar, laminaceous, toothed on 



