GENERA OF SHELLS. 57 



CLIO. Has no shell. 



CLEODORA. Shell gelatino-cartilaginous, 

 transparent, like an inverted pyramid, or lan- 

 ceolate, truncated and open superiorly. 



C. pyramidata, caudata. 



LIMACINA. Shell thin, fragile, papyraceous, 

 spiral ; whorls in discoidal order, like planorbis. 



L. helicialis. 



CYMBULIA. Shell gelatino-cartilaginous, very 

 transparent, crystalline, oblong, shoe-shaped, 

 truncated at summit ; aperture lateral and in- 

 terior. 



C. Peronii. 



PNEUMODERMON. No shell. 



ORDER II. GASTEROPODA. 



Some are naked, others have a dorsal shell, not envelop* 

 ing ; others again have a shell more or less hidden in 

 a mantle. 



TRTTONIANA. 



Without shells, either external or internal. 



PHYLLIDIANA. 



Some are without shells either external or internal ; others 

 are wholly or partly covered by a shell, sometimes com- 

 posed of one single piece, sometimes of a range of mova- 

 ble and distinct pieces. 



PHYLLIDIA. No shell. 



CHITONELLUS. Shell multivalve ; valves al^ 

 ternate, mostly longitudinal, united to each 



