14 



Yar. III. 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 natter than S. corneum, especially when young, 

 more solid and more strongly striated. 



3. S. OVALE (oval). 



Oblong, thin, compressed, semi-transparent, dull, pale 

 grey ; strice faint, concentric \ beaks central j 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 S. corneum, thin, glossy, 

 semi-transparent, regularly striate concentrically; beaks 



