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 IL — GASTEROPODA. 



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

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

 a mantle. 



TRITONIANA. 



Without shells, either external or internal. 

 PHYLLIDIANA. 

 Some are toithout shells either external or internai; others 

 arc wholly or partlij covered by a shell, sometimes com- 

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

 ble and distinct pieces. 



Phyllidia. No shell. 

 Chitonellus. Shell multivalve ; valves al- 

 ternate, mostly longitudinal, united to each 



