PTEROPODA AND GASTROPODA. 573 



Class GASTROPODA. 



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



A. MON(ECIA. 



Order Apneusta. 



No distinct respiratory organs : no shell. 



Genera Actceon, Rhodope, Zephyrina, Tergipes, Eolidma, jEolis. 



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. 



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

 drogynous. 



Genera Tornatella, Bidla, Bullcea, Doridium, Gastropteron, 

 Aplysia, Dolabello, Notarchus, Icarus, Lobiger, Pleu- 

 rohranchus. 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 epiphragm during hybernation. 



Genera Helix, Vitriiia, Succinea, Bulinus, Achatina, Pupa, 

 Clausilia. 



Family LimacidcB. Shell rudimental, internal. 



Genera Limax, Arion, Parmacella, Testacella. 

 Family Oncidiadce. No shell. 



Genera Oncidium, Vaginulus, 



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

 Genera Limnea, Physa, Ancylus, Planorbis. 



Family AuricuUdce. Shell with a horny epidermis. 

 Genera Auricida, Co7iovulus, Carychium. 



