112 SYSTEMATIC ARRANGEMENT 



lateral 3 • 3, converging. Eyes sessile, or on very short tubercles on 

 the outer side of the base of the tentacles. Tentacles subulate. 

 Neck with a small lobe on each side. Male organ at the base of the 

 right tentacle. Gill formed of three ranges of filaments. Mantle 

 simple in front. Shell free, spiral, ovate, covered with a thin, trans- 

 parent, or thick, hard, olive, polished periostraca ; aperture ovate. 

 Peristome continuous. Operculum annular. 

 Fluviatile. 



The family has much the external appearance of the Apple-snails 

 (Ampullariadce), also river shells chiefly found in warm climates, 

 and which likewise have an annular operculum ; but these have 

 pedicelled eyes, very long tentacles, and a long appendage to the 

 forehead. 



§ Operculum horny, thin. 



1. Viviparus. 



Shell ovate, thin, banded. Periostraca thin, with hairy bands. 

 Peristome thin. Operculum horny, nucleus near the middle of inner 

 side. — Adams, Gen. Moll. t. 36. f. 1 a. 



1 . V. vulgaris, t. 124. f. 3 ?, 6, 9 ; Forbes fy Hanley, B. M. t. H.H. 



f. 2. 



2. V. bengalensis, t. 124. f. 2. 



3. V. ampullariformis, t. ]27 a. f. 5. 



4. V. decisus, t. 310. f. 10. 



5. V. fasciatus, Adams, Gen. Moll. t. 36. f. 1. 



The lingual band of Viviparus strong, slender, elongate. Teeth 

 3 • 1 • 3 (fig. 58), laminal, longitudinal, subovate, apex recurved ; 



Fig. 58. — Teeth of Viviparus vulgaris. 



middle lobed, obtusely toothed on each side ; lateral nearly similar, 

 laminaceous, slightly curved ; apical edged, denticulately lobed on 

 each side ; the inner shorter and broader, the outer winged on the 

 outer edge. — Lovhn. 



The gills of Paludina are formed of a series of triangular plates 

 attached to and forming an oblique line down the left side of the 

 branchial cavity. There is a similar ridge in the cavity to that in 

 Ampul I a rid, but the ridge up the back of the neck runs more to the 

 left, and there is a fat mass between it and the right side of the body. 



