OF MOLLUSCA. 115 



The Bithiniee are oviparous, their eggs being deposited in oblong 

 groups, like the Lymnei or pond-snails, on the stems and leaves of 

 freshwater plants. 



d. Gills laminar, forming an oblique line across the mantle-cavity ; 

 laminae elongate, linear. Lingual band elongate, winged at the 

 sides in front. Shell free, subspiral ; mouth large, expanded ; 

 of young regularly spiral. Animal sedentary, rarely changing 

 its place (pp. 78, 86, 95). 



* Operculum none. 



Fam. XX. CALYPTRJSADiE. 



Shell subspiral, depressed or subconic. The front of the body- 

 produced, depressed, rather dilated on the sides. Rostrum short, 

 scarcely produced, depressed. Mouth erect, small, terminal. Ten- 

 tacles subulate, short, at the base of the rostrum. Gills of rather 

 short filaments. Foot small, expanded, well separated from the 

 mantle. Lingual membrane elongate, winged on each side in front, 

 the wings being united beneath. Teeth 3 ■ 1 ■ 3 ; the central simple, 

 recurved, toothed at the tip ; the inner lateral broad, with a trian- 

 gular toothed tip ; the outer claw-like ; the second lateral minutely 

 toothed at the end. 



The eggs are enclosed in a thin membranous bag in small groups 

 under the foot of the animal. The shell in the egg is subglobular, 

 of one or two gradually enlarging whorls, which rapidly enlarge as 

 soon as the animal is hatched. 



a. Crepidulina. Foot naked. Shell subspiral. Body and shell 

 depressed horizontally ; apex of the shell sublateral, subpqs- 

 terior ; inner plates of the shell horizontal. 



1. Crepidula. 



Shell Oblong, convex ; inner lip transverse, concave beneath. 



1. C. costata, Fig. Moll. t. 58*. f. 2. 



2. C. maculata, t. 58*. f. 4. 



3. C. aculeata, Adams, Gen. Moll. t. 40. f. 7, t. 58*. f. 5. 



4. C. patagonica, t. 58*. f. 6. 



5. C. Adolphsei, t. 130. f. 4 (animal in shell the wrong way). 



Central tooth of Crepidula unguiformis compressed ; cutting edge 

 Fig. 60. — Teeth of Crepidula unguiformis. 



produced, triangular, with one sharp point, serrated on both sides ; 



i 2 



