MOLLUSCA. PULMONIFERA. 273 



AQUATICA. 



Tentacula two, usually Jlat, with eyes at the base. Respire at 

 the surface of the water. Sexes united ; spawn deposited 

 under water on aquatic plants. Phytivorous. 



a. Shell spiral. 



b. Shell turrited. 



c. Whorls dextral. 

 Limnea. 

 Assiminia. 



cc. Whorls sinistral. 

 Physa. 

 Aplexa. 



bb. Shell discoid. Tentacula filiform. 

 Planorbis. 

 Segmentina. 



aa. Shell simple, conical. 

 Ancylus. 



Gen. XXIX. LIMNEA. — Aperture of the shell longitudinal ; 

 the outer lip, in bending in on the pillar, forming an oblique 

 entering: fold. Tentacula lanceolate. 

 * Shell turrited. 



— 105. L. stagnalis. — Whorls six or seven ; the last large in 



proportion. 



Bucc. longum 6 spirarum, List. An. Ang. 137- Conch. Tab. 123, f. 21. — 

 B. stag. Mull. Verm. ii. 132. — Helix stag. Penn. Brit. Zool. iv. 138. 

 tab. lxxxvi. f. 130. Mont. Test. Brit. 367— In lakes and stagnant 

 waters. 



Length nearly 2 inches ; brownish, translucent ; lines of growth distinct, 

 with numerous longitudinal wrinkles ; the upper whorls smooth ; the separat- 

 ing line distinct. Lister mentions having seen a variety with branched ten- 

 tacula. 



106. L. fragilis. — Whorls six, diaphanous ; upper whorls 

 with a shallow oblique separating line. 



Helix fragilis, Mont. Test. Brit. 369, tab. xvi. f. 7-— In canals in Eng- 

 land and Ireland. 



This species is less than the preceding ; the aperture is narrower ; the 

 spires increase more gradually, and the first formed ones are less rounded. 

 It was observed by Montagu, in the Kennet and Avon Canal, Wiltshire. It 

 likewise occurs in the Grand Canal near Dublin. The specimens I possess 

 were sent to me by Dr Leach, from the Croydon Canal. 



107. L. detrita. — Shell thick, obtusely conical, of six whorls, 

 with a brown band along the line of separation. 



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