462 MANTJAL OF THE MOLLUSCA. 



Shell thin, ventricose, nearly equilateral; cardinal teeth 2.1, 

 minute, laterals 1 — 1 : 2 — 2, elongated, compressed. 



Animal ovo -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 infernal branchiae, they 

 are few in number and very unequal in size ; a full-grown 

 O. cornea has about 6 in each gill ; the largest being -g^ to ^ the 

 length of the parent. The young Cydades and Fisidia 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. 



I 



Fig. 257. Pisidium amnicum, ■§, with its foot protruded. 



Sub-genera, Pisidium, Pfr. P. amnicum, PI. 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 conHuent. 



Distrihution, 60 species. United States, South America, 

 Greenland, Norway, Sicily, Algeria, Cape, India, Caspian, 

 Britain. 



Fossil, 38 species. "Wealden — . Europe. 



Ctrena, Lamarck. 



Etymology, Gyrene, a nymph. 



Ty^e, C. cyprinoides, PI. XIX., Pig. 20. 



Shell oval, strong, covered with thick, rough epidermis ; 

 ligament thick and prominent; hinge-teeth 3.3, laterals 1 — 1 

 in each valve ; 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. C. consobrina, PI. XIX., 

 Pig. 21. Shell orbicular, concentrically furrowed, epiderrais 

 polished ; lateral teeth elongated, striated p-cross. 



