CONCHIFERA. 297 



Cyclas, Bi'uguiere. 



Etym. KuUas, orbicular. Type, C. Cornea. PI. XIX. fig. 17. 



Syn. Sphserium, Scop. Pisura, Muhlf. (not L.) Musculium, Link. 



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 internal branchiae, tbey are few iu 

 number and very unequal in size ; a full-grown C. cornea has about 6 in 

 each gill ; the largest being ^ to i the length of the parent. The young 

 Cy eludes and Pisidia are very active, climbing about submerged plants and 

 often suspending themselves by byssal threads ; the striated gills and pulsat- 

 ing heart are easily seen through the shell. 



Fig 216. Pisidium amnicum, -f. with its foot protruded. 



Sub-gemis, Pisidium, Pfr. P. amnicum, PI. XIX. fig. 18. Shell ine- 

 quilateral, anterior side longest; teeth stronger than in Cyclas. Animal 

 with a single, small, escurreut siphon; branchial and pedal orifices confluent, 



Disir. 30 sp. U. States, S. America, Greenland, Norway, Sicily, Algeria, 

 Cape, India, Caspian. 



Fossil, 35 sp. Wealdeu — . Europe. 



Cykena, Lamarck. 



Ety^n. Gyrene, a nymph. %;^, C. cyprinoides, PI. XIX. fig. 20. 



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

 and prominent; liiuge-teeth 3.8, 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 lanceolate ; foot strong, tongue-shaped. 



Section, Corhicula, Muhlf. C. consolrma, PI. XIX. fig, 21. Shell 

 orbicular, concentrically furrowed, epidermis polished; lateral teeth elon- 

 gated, striated across. 



Bistr. 25 sp. Tropical America (eastern) ; Egypt, India, China, Australia, 



