90 systematic arrangement 



4. Hydrobia. 



Shell ovate ; spire conical ; mouth ovate; lips simple. Operculum 

 ovate, of few whorls. 



1. H. parvula, t. 123. f. 7. 



2. H.? subumbilicata, t. 123. f. 3. 



5. Amnicola. 



Shell ovate, conic, perforated ; whorls rounded ; mouth roundish 

 ovate ; lips simple. Operculum oblong, of four or five rapidly en- 

 larging whorls. — Adams, Gen. Moll. t. 35. f. 11 a. 



1. A. porata. 



Head of Amnicola protruding beyond the foot. Tentacles short, 

 filiform, unequal ; eyes at the outer side of the base. Foot elongated, 

 rounded behind, with produced angles in front. 



Living crawling on stones, sticks and aquatic plants in rivers and 

 ponds, and often swimming in an inverted position on the surface of 

 the water. The head precedes the animal as it walks. The tentacles 

 are unequal in length. They are oviparous. 



6. Nematura. 

 Shell oblong, compressed ; spire elongate ; peristome continuous ; 

 mouth roundish, contracted. Operculum ovate, thick, shelly; whorls 

 three, rapidly enlarging. — Adams, Gen. Moll. t. 36. f. 5 a, b. Flu- 

 viatile. 



1. N. Deltse, Adams, Gen. Moll. t. 36. f. 5 (shell only). 



2. N. polita. 



They live in rivers and streams in the East, and are found adhering 

 to the under surface of dead floating leaves. 



7. B.1SELLA. 



Eyes rather above the base of the tentacles. Shell trochiform, 

 imperforated ; whorls angular, flattened in front ; aperture rhombic, 

 smooth internally. Operculum ovate. — Adams, Gen. Moll. t. 33 . f. 5 a. 



Fig. 50. — Teeth of Risella melanostoma. 



Lingual membrane narrow, elongate. Teeth very transparent, 

 subsimilar, tridentate (fig. 50). 



