75 
KEY TO GENERA AND SPECIES 
The identification of some of the species is not easy, and 
the following key is given to facilitate their recognition. This 
key is based solely on the shell characters. As noted elsewhere 
the foundation of many of the genera, and their grouping into 
families, rests upon the anatomy of the living animal; hence a 
key based upon shell characters alone cannot be considered 
complete, nor in many cases very satisfactory. From the shell 
characters alone Lymnaea and Planorbis could not be grouped 
into the Lymnaeidse. However, only such a key can be used 
for the fossil forms. 
A. Shell bivalve, two valves symmetrical, united by a dorsal hinge. 
a. Shell small, two sets of cardinal teeth in each valve. 
Family Sphaeriida 
Gl. More or less rounded, or ovate, seldom trigonal in outline, 
usually not less than 6 mm. in length. 
Genus Sphaerium 
1. Shell large, oval, beaks nearly in a median 
position S. simile 
2. Shell inequilateral, solid; umbones small, 
almost smooth S. striatinum 
3. Shell solid, more or less trigonal in outline; 
umbones anterior to centre of shell solidulum 
G2. Shell small, fragile, rounded to trigonal in outline, beaks 
posterior of middle of shell, mostly less than 4 mm. in 
. length. 
Genus Pisidium 
4. Shell small, more or less rounded, beaks in-. 
conspicuous nearly in middle of shell. Shell 
covered with fine growth lines P. abditum 
5. Shell small, trigonal, umbones very much 
posterior to middle of shell, anterior end 
produced P. compressum 
B. Shell univalve, usually coiled such as the snails 
b. Shell depressed, globose 
Family Helicida 
G3. Genus Polygyra 
6. Shell large, imperforate; tooth lacking; peri- 
stome widely reflected, white P. albolabris 
7. Shell depressed, minute growth lines, peri- 
stome simple, prominent parietal tooth P. monodon 
c. Ribbed or striated; umbilicated; aperture simple 
