is uppermost. This is a convenience for understanding the shell, but is the opposite of the way the shell is actually carried in life. The side of the shell which is in fact the spire (a sunken spire) faces down in the living animal, contrary to what is the case in almost all other shelled gastropods. Because the shell is carried "upside down" like this, the aperture of the shell is angled to face downwards also, so the aperture faces a little towards the spire, not away from it, as is usually the case in other shelled gastropods. Habitat Most species of planorbids live only in fresh water, such as ponds, lakes, and slow moving rivers. A minority of species are able to survive in brackish water. Geological history Ancestors of ramshorn snails are known with certainty since the Jurassic period. Modern taxa developed in the Jurassic. Geographical distribution Species in this family occur worldwide. In Northwest Europe about 20 species are known (including non-indigenous species). In this region, various extinct taxa are known to have occurred, starting in the Jurassic period. References Gittenberger, E., Janssen, A.W., Kuijper, W.J., Kuiper, J.G.J., Meijer, T., Velde, G. van der & Vries, J.N. de (1998) De Nederlandse zoetwatermollusken. Recente en fossiele weekdieren uit zoet en brak water Nederlandse Fauna 2. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & EIS-Nederland, Leiden, 288 pp. Fischer, P.H. (1880-1887) Manuel de Conchyliologie et de Paléontologie conchyliologique ou histoire naturelle des Mollusques vivants et fossiles suivi d'un appendice sur les Brachiopodes par D. Oehlert. – XXIV + 1369 pp. (German) Thiele, J. (1929-1935) Handbuch der Systematischen Weichtierkunde. Jena, (1), 1-376, 1929; (2), 377-778, 1931; 779-1022, 1934; 1023-1134, 1935. Vaught, K.C. (1989) A classification of living mollusca. American Malacologists Inc., Melbourne USA, & 0-915826-22-6, 195 pp. (German) Wenz, W. (1923-1930) Gastropoda extramarina tertiaria. Fossilium Catalogus I. (4 vols.), 3387 pp. (German) Zilch, A. (1959-1960) Euthyneura. In: H. Schindewolf (ed.), Handbuch der Paläozoologie, 6(2): pp. I-XII + 1-834. External links NCBI Taxonomy Browser: Planorbidae Systema naturae 2000 (classification) - Taxon: Planorbidae ITIS: Planorbidae http://mkohl1.net/Planorbidae.html Frank Collins Baker, The Molluscan Family Planorbidae, The University of Illinois press, Urbana, 1945 Gastropod families Taxa named by Constantine Samuel Rafinesque