604 GASTROPODA. [Pulmonata. 



to the surface, and creep, as it were, reversed on the surface of the 

 water. They lay their eggs in consistent transparent gelatinous 

 masses on the leaves and stems of water-plants, or on stones. 

 Fossil they first appear in the Lias. 



KEY TO GENERA. 



A. Shell wholly external, with a pointed spire . . . . LYMXOKA. 



B. Shell in great measure covered by the mantle, globular, with 



a very short spire . . . . . . . . . . AMPHIPEPLEA. 



Genus 1. LYMNOEA, Lamarck, 1799. 



Limnoi/a, Lam., Mem. Soc. Nat. Hist. Paris, 1799, 75. Type : Helix 

 stagnalis, L. Limncea. Lam., 1801. Limnaeus, Drap. Lymn-us, Montf. 



Animal spiral, not covering the shell when emerging from it ; head 

 large ; tentacles triangular, flattened ; eyes sessile, situated at the 

 inner bases of the tentacles ; pulmonary orifice protected by a pro- 

 jecting lobe. Jaw composed of 3 segments. Central tooth of radula 

 very small ; laterals bicuspidate ; marginals multicuspidate. 



Shell spiral, thin, and horny, mostly dextral ; spire pointed ; 

 aperture oval, ample, rounded in front, the margins united by a thin 

 callosity ; peristome thin, sharp ; columella more or less twisted. 



Distribution. In fresh water of all latitudes, from Greenland to 

 the Straits of Magellan ; in lakes of high altitude ; and in all different 

 kinds of water, even brackish. One species (L. abyssicola) has been 

 dredged alive in the Lake of Geneva in depths from 75 to 250 metres. 



The oldest known forms are Jurassic. 



KEY TO SPECIES. 



A. Height of spire a little less than that of the aperture. 



a. Shell elongate-oval, the last whorl slightly ventricose ; a 



small umbilical chink . . . . . . . . Alfredi. 



aa. Shell oblong, the last whorl moderately convex, high ; no 



umbilical chink . . . . . . . . . . pusilla. 



B. Height of spire equal to that of the aperture. Shell elongate-oval, 



the last whorl not ventricose ; columella with a slight median 



fold ; a narrow umbilical chink . . . . . . . . tenella. 



C. Height of spire two-thirds that of the aperture ; body-whorl with 



4-5 obsolete spiral ridges, ventricose . . . . . . leptosoma. 



D. Height of spire half that of the aperture. Shell oblong-conic, 



with downy hairs ; columella arcuate . . . . . . tomentosa. 



1. Lymnoea Alfredi, Suter, 1890. Plate 24, fig. 10. 



Limncea Alfredi, Suter, T.N.Z.L, xxii, 1889 (1890), 229. pi. 15, f. 17; 

 Hedley and Suter, P.L.S. N.S.W. (2), vii, 1893, 626. L. Tenisoni, 

 Cless.. var. Alfredi, Sut., J. de Conch., xli, 1893 (1894), 230. 



Shell small, thin and fragile, elongated oval, semitransparent, 

 glossy. The only sculpture consists of very fine and close flexuous 

 growth-hues, with an occasional period of rest. Colour horn-yellow. 

 Epidermis very thin, membranous. Spire elevated conic, its height 

 very little less than that of the aperture ; outlines slightly convex. 



