14 SPHHRIDZE. 
Var. ITI. Scaldiana. Ovate, and paler than type. 
Var. IV. Pisidioides. Somewhat triangular. 
2. S. RIvicoLa (inhabitant of streams). 
Oval, solid, opaque, glossy, greenish or yellow when 
young; striae evenly concentric; ligament short, but con- 
spicuous ; scars distinct; teeth as in last species, but larger. 
It inhabits canals and slow rivers in various parts of 
England, and near Dublin. 
This is the largest species in the genus. It is also 
much flatter than S. corneum, especially when young, 
more solid and more strongly striated. 
3. 8. OVALE (oval). 
Oblong, thin, compressed, semi-transparent, dull, pale 
grey ; strice faint, concentric ; beaks central ; ligament long; 
hinge straight on posterior, incurved on anterior side ; teeth 
small; scars distinct. 
This is a local species, being found in ponds and 
canals in Lancashire, Yorkshire, and near Birming- 
ham. Its oblong shape, thin delicate appearance, pale 
colour, and straight hinge, prevent its being mistaken 
for any others of this genus. 
4, S. LacustRE (inhabiting lakes). 
Round, more compressed than 8. corneum, thin, glossy, 
semi-transparent, regularly striate concentrically; beaks 
