34 VALVATID^. 



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- 

 ino; whorls, which have a raised membranaceous 

 outer edge, forming a spiral ridge on the outer 

 surface (f. 9. 6.). 

 The shells are known from Paludinte by the shelly 



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



the proximate whorls. 



1. Valvata Midler. (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 piscinalis. Stream Valve-shell. — 

 Shell globular, with an elevated obtuse spire, 

 and a deep central umbilicus, (t. 10. f. 114.). 



The lingual plate of V. piscinalis is elongate; 

 Fig. 7. the central tooth sub- 



quadrate, base pro- 

 duced on both sides, 

 incurved, cutting edge 

 triangular, the middle 

 Teeth of Valvata. \^ ^^-^j^ ^ pectinated 



a, central ; b, c, d, lateral. i • i i , 



lobe on each side ; late- 

 ral teeth somewhat similar, laminaceous, toothed on 



