PTEEOPODA AND GASTROPODA. 573 



Class GASTROPODA. 

 Encephalous mollusks, with a locomotive ventral disc, or foot. 



A. MONCECIA. 



Order Apneusta. 



No distinct respiratory organs : no shell. 



Genera AcUeon, Rhodope, Zephyrina^ Tergipes, Eolidina, ^olis. 



Order Nudibranchiata. 



Branchiae extending more or less freely from various parts of the 

 body. 



Genera Scyllcea, Tritonia, Doris, Thetis, Polycera. 



Order Inferobranchiata. 



Branchiae at the lower part of the sides of the body, between the 

 foot and mantle. 



Genera Phyllidia, Diphyllidia, Ancylus. 



Order Tectibranchiata. 



Branchiae covered by the shell or mantle : a shell in most. An- 

 drogynous. 



Genera Tomatella, Bulla, Bullcea, Doridium^ Gastropteron, 

 Aplysia, Dolahella, Nolarchus, Icarus^ Lobiger, Fleu- 

 Tobranchus, Umbrella. 



Order Pulmonata. 



Part of the mantle-cavity forming a vascular air-sac or lung. 



Family Helicidce. Shell external, usually well developed, closed 

 by an epiphragra during hybernation. 



Genera Helix, Vitrina, Succinea, Bulinus, Achatina, Pupa, 

 Clatisilia. 

 Family Limacidce. Shell rudimental, internal. 



Genera Limax, Avion, Pamiacella, Testacella. 

 Family Oncidiad(S. No shell. 

 Genera Oncidium, Vaginulus. 



Family Limneidce. Shell thin, horn-like, well developed. 

 Genera Limnea, Physa, Ancylus, Planorbis. 



Family Auriculidce. Shell with a horny epidermis. 

 Genera Auricula, Conowlus, Caryckium. 



