177 



Figure. 



Paludomus undatus. PL 18. Fig. 100. Shell, showing its globose 

 Natica-like form. 



Family 13. LYMN^EANA. 



Shell ; thin, mostly transparent, smooth ; aperture sometimes very 

 large, sometimes very small, with the lip simple. 



The well-known pond and ditch Snails which constitute this family dwell 

 in fresh-water as their proper element, but have the capacity of breathing 

 air, to provide against the emergency of their places of habitation becoming 

 dried up. They are accustomed to rise to the surface of the water to 

 breathe and to take in a supply of air, and are furnished with the means of 

 retaining it for some considerable time.* 



In aquaria they have been observed to lower themselves from plants at 

 the water level, by a mucous thread, and to re-ascend by it. The genera 

 are: — 



ANCYLUS. LYMNiEA. PhYSA. PlANORBIS. 



Genus 1. ANCYLUS, Geoff roy. 



Animal ; with a broad head, tentacles triangular, with eyes at 



their base ; mantle simple ; disk ovate, large. 

 Shell ; thin, Limpet-shaped, with the vertex rather sharp, incurved 



posteriorly ; aperture oval, very large, simple at the margin. 



The Limpet form of the little Ancylus led to many errors among concho- 

 logists before the discovery of its true place in the system. So unlike Lymnaa 

 and Planorbis in the absence of convoluted whorls, no one could have sup- 

 posed the shell to have any immediate relationship with those genera. But 

 the animal, observed by Guilding and De Ferussac, was found, notwithstand- 

 ing the difference of its conchological characters, to be nearly the same, 

 and to be affected by the same peculiarities as regards its habits and mode 



* Professor Edward Forbes describes that when ascending in sunny weather, the Lymnfeantc 

 creep, as it were, reversed on the surface, as if the film of water in immediate contact with the 

 air was in a different condition from the fluid beneath, and served as a floor or ceiling on which 

 these mollusks progress. They lay their eggs, he continues, in consistent, transparent, gelatinous 

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



2 A 



