BIVALVES. 
35 
itself witli the shell immersed and inverted, by 
glutinous threads, which are spun by the foot, or 
it may even be observed gliding along the surface 
of the water in an inverted position. It is very 
active, and climbs the submerged plants with 
great facility, among which it is usually found; 
it can suspend itself by its glutinous threads, 
which are of the nature of a byssus, as in the 
Driessena and Mytilus. 
It thrives well and breeds in confinement; 
the fry are hatched in the gills, are but few in 
number (each gill containing not more than, 
about six), and of different sizes, the largest va¬ 
rying from one-eighth to one-fourth the length 
of the parent. 
The larv83 of a fluke (. Amphistoma subclava - 
turn) have been found on the surface of the body 
of this and other species of Gyclas , as also on 
the coil-shells ( Planorbis ). It is a newer Ter¬ 
tiary species. 
Oyclas pisidioides (PL III., fig. 9) is a recent 
addition to science; it was discovered, in 1856, 
in the Paddington Canal, near Kensal Green, 
T ondon. It has much the appearance of a large 
Pisidium ; hence its specific name. The adult 
shells are six lines long, five wide, and four thick. 
It is distinguished from G . cornea by its subtri- 
angular shell, which is somewhat produced behind 
and slightly wrinkled concentrically. 
