BIVALVES. 33 
The family contains two British genera, Cyclas 
and Pisidium; the respiratory siphons are two 
in number in Oyclas (PI. IIIL., fig. 17); there is 
only one (fig. 3, s) in Pisidiwm, the branchial 
and pedal orifices being confluent (b). The shells 
of Pisidiwm are inequilateral, those of Cyclas 
rounded, and more or less equilateral. 
Genus CyYcLAs. 
There are five British species of this genus, 
the largest of which is 
CycLas rivicota—(the River Cyclas) (Pl. II1., 
fig. 17).—The shell of this species is distin- 
guished by its great size, its more oval form, 
the strong concentric ridges on the shell, and 
the prominent ligament. The oval equilateral 
shell is ten lines in length, seven in width, 
and five in thickness; the epidermis is thick, of 
a glossy reddish or greenish-brown, with two 
or three darker bands, strongly striated concen- 
trically; the interior of the valves is whitish, 
with a bluish or yellowish tinge. 
This species burrows in the mud of canals 
and slow-running rivers, in the southern and 
midland counties of England: it is plentiful m 
the docks and canals about London; it occurs in 
the Thames, above Chelsea; in the Medway at 
Maidstone; Kennet and Avon Canal, Wilts; 
Severn, at Wainlode; Tewkesbury; Newent, 
D 
