462 MANUAL OF THE MOLLUSCA. 
Shell thin, ventricose, nearly equilateral; cardinal teeth 2.1, 
minute, laterals 1—1: 2—2, elongated, compressed. 
Animal oyo-viviparous; siphons partly united, anal shortest, 
orifices plain, gills very large, the outer smallest, with a dorsal 
flap; palpi small and pointed. 
The fry of Cyclas are hatched in the internal branchiee, they 
are few in number and yery unequal in size; a full-grown 
C. cornea has about 6 in each gill; the largest being ¢ to 4 the 
length of the parent. The young Cyclades and Pisidia are very 
active, climbing about submerged plants and often suspending 
themselves by byssal threads; the stiiated gills and pulsating 
heart are easily seen through the shell. 
Fig. 257. Pisidium amnicum, %, with its foot protruded, 
Sub-genera, Pisidium, Pfr. P. amnicum, Pl. XIX., Fig. 18. 
Shell inequilateral, anterior side longest; teeth stronger than in 
Cyclas. Animal with a single, small, excurrent siphon; bran- 
chial and pedal orifices confluent. 
Distribution, 60 species. United States, South America, 
Greenland, Norway, Sicily, Algeria, Cape, India, Caspian, 
Britain. 
Fossil, 38 species. Wealden—. Europe. 
CyrEena, Lamarck. 
Etymology, Cyrene, a nymph. 
Type, C. cyprinoides, Pl. XIX., Fig. 20. 
Shell oval, strong, covered with thick, rough epidermis; 
ligament thick and prominent; hinge-teeth 3.3, laterals 1—1 
in each yalye; pallial line slightly sinuated. . 
Animal (of type) with the mantle open in front and below, 
margins plain; siphons short, orifices fringed; gills unequal, 
square in front, plaited, inner lamina free at base ; palpi lanceo- 
late; foot strong, tongue-shaped. 
Sub-genera, Corbicula, Muhlf. CC. consobrina, Pl. XIX., 
Fig. 21. Shell orbicular, concentrically furrowed, epiderrais 
polished ; lateral teeth elongated, striated across. 
