114 SYSTEMATIC ARRANGEMENT 



2. RlVULINA. 



Shell ovate, conical, smooth ; upper whorls dotted or lined with 

 brown. Peritreme continuous, with a slight depression behind the 

 columella in place of an umbilicus. " Operculum like Paludina" 



1. R. modicella, Lea, P. Z. S. 1850, 197. 



2. R. zeylanica. 



3. Paludomus. 



Shell ovate, conical, solid, smooth, with granulated ribs. Oper- 

 culum horny, annular ; nucleus sub central, near the front or inner 

 edge. — Adams, Gen. Moll. t. 36. f. 2 a. 



In ponds and slow rivers. 



1. P. conicus, Gray, P.Z. S. 1849. 



2. P. ?spurcus, t. 127 a. f. 4, edge of mantle festooned. Is it a 



Melania ? 



4. TlNALIA. 



Shell semiglobose, costate, nodulose. Mouth very large, ovate. 

 Operculum horny, annular ; nucleus marginal, near the front of the 

 outer edge. — Adams, Gen. Moll. t. 36. f. 3. Mountain streams. 



1. T. aculeata. 



§§ Operculum horny, with an internal shelly coat. Oviparous. 



5. BlTHINIA. 



Shell ovate, thin, with a thin periostraca ; whorls convex. Peri- 

 stome thickened internally. Operculum with a shelly internal coat ; 

 nucleus subcentral. 



1. B. tentaculata, t. 124. f. 8; Forbes fy Hanley, B. M. t. H. H. 



f. 3 ; Adams, Gen. Moll. t. 36. f. 4. 



2. B. viridis, t. 124. f. 7. 



3. B. Leachii, Forbes fy Hanley, B. M. t. H.H. f. 4. 



4. B. sulcata = Valvata sulcata. 



M. Moquin-Tandon observes, Bithinia differs from Paludina thus : 

 The head is narrow and does not exceed the plane of the foot ; it 

 has no jaws ; the eyes are perfectly sessile, rather behind the base of 

 the tentacles ; the tentacles are always equal ; the gills are formed 

 of folds placed in one series ; the stomach is furnished with carti- 

 laginous stylets ; the male organ is bifid, always exterior, and situated 

 behind and on the outside of the right tentacles, and it is destitute 

 of the large glandular pouch which yields the red colour of the Palu- 

 dince. — Petit, Journ. Conch. 1851, 245. 



In Bithinia the gills consist of only a single series of transverse 

 parallel flexuous folds attached to the surface of the branchial cavity, 

 which are never visible externally ; the fold narrows at the two ends 

 of the apparatus ; the penis is placed on the base of the right tentacle, 

 as in Puludina. 



