INVERTEBRATE PALAEONTOLOGY. 533 



5. limnophysa, Fitzinger (zr Stagnicola, Leach, and Galba, Schranck). 



Shell subovatc; spire conical and about as long as the aperture; 



volutions convex: lip nol expanded. — (L. palustris, Miiller.) 



6. omphiscola, Raf. (= Leptolimnea Swainson). 



►Shell subcylindrical; spire produced, thick; aperture compar- 

 atively small. — (L. glabra, Midler.) 



7. acella, llaldeman. 



Shell very slender ; spire acuminate, longer than the aperture; 

 volutions flattened; suture very oblique, aperture expanded ; inner lip 

 without a told. — (L. gracilis, Say.) 



8. pleurolimnjea, Meek. 



Shell differing from the last in having small, regular, surface- 

 costse parallel to the lines of growth, and aperture narrowed or sub- 

 angular, instead of rounded anteriorly. — (L. tenuicosttita, Meek and 

 Hay den.) 

 As is well known to all conchologists and collectors of shells, the species 

 of the genus Limncea are entirely confined to fresh waters, being found in 

 ponds and other bodies of still water. They are vegetable-feeders, and have 

 a curious habit of floating at times, with the expanded foot upward, even 

 with the surface of the water, and the shell downward, during which they 

 move along slowly by an undulatory motion of the foot. As noticed by H. 

 and A. Adams and others, when the ponds in which they live become dried 

 up by evaporation during droughts, they bury themselves in the mud, 

 secrete a kind of rib within the outer lip, and, close the aperture by a sort of 

 false operculum, like the Helicidce during certain seasons of inactivity. 



The aperture of the curious type of the section Pleurolimnma has not 

 yet been seen, and we cannot be positively sure that this shell will certainly 

 fall within the genus Limncea. It differs from all the other forms undoubt- 

 edly belonging to it in the possession of costse, and in having the aperture 

 (judging from the form of the body-volution) narrowed, or subangularin front. 

 This genus is widely distributed at the present time, being found in this 

 country as well as in Europe, China, and various other parts of the world. 

 Its geographical distribution was also apparently as great during the Tertiary- 

 period. The oldest species yet known, I believe, occurs in the Upper Oolite 

 of England. It has also been found in the Wealden beds, and in the fresh- 

 water deposits throughout the Tertiary rocks. 



