MOLLUSCA: GASTEROPODA. 319 



Pant. 2. Bullidm. Shell convoluted, thin ; spire small or concealed, 



lip sharp. Animal more or less investing the shell. III. Gen. 



Bulla, Cylichtia, Philine. 

 Fam. 3. Aplysiadce. Shell absent, or rudimentary and concealed 



by the mantle. Animal slug - like, witli extensive side - lobes, 



(epipodia), reflected over the back and shell. 111. Gen. Aplysia, 



Dolabella, 

 Fam. 4. Pleurobranchidcs. Shell patelliform, or concealed, rarely 



wanting. Mantle or shell covering the back of the animal. 111. 



Gen. Pleurobranchtis, Umbrella, Tylodina. 

 Fam. 5. PhyllidiadiB. Animal shell-less, covered by a mantle. XIL 



Gen, Phyllidia, Diphyllidia. 



Section b. Nudibranchiata. Animal destitute of a shell in the adult con' 

 dition. Branchiee external, on the back or sides of the body. 

 Fam. 6. Dorida. 111. Gen. Doris, 

 Fam.. 7. Tritoniada. 111. Gen. Tritonia, Scyllcea. 

 Fam. 8. ^olidce. 111. Gen. jEolis, Glaucus. 

 Fam. 9. PhyllirhoidcE. Gen. Phyllirhoe. 

 Fam. 10. Elysiada. 111. Gen. Elysia, Actconia. 



OjSiDER III. NucLEOBRANCHiATA Or Heteropoda. Shell present or 

 absent. Animal free-swimming and pelagic, with a fin-like tail, 

 or a flattened ventral fin. 



Fam. I. Firolida. Body large ; branchiae exposed on the back, or 

 covered by a small hyaline shell ; locomotion by means of a ven- 

 tral fin and a tail-fin. 111. Gen. Carinaria, Firola. 



Fam. 2. Atlantidce. Animal furnished with a well-developed shell 

 into which it can retire. Branchiae contained in a dorsal mantle- 

 cavity. Shell symmetrical, discoidal, sometimes with an oper- 

 culum. 111. Gen. Atlanta, Bellerophon, Maclurea. 



Section B. Pulmonifera. Respiration aerial, by means of a pulmonaiy 

 chamber. 

 Division I. Inoperculata. Shell not provided with an operculum, 



Fam, I. Helicida. Shell external, capable of containing the whole 



animal. 111. Gen. Helix, Bulimus, Clausilia, Pupa. 

 Fam, 2, Limacidce. Shell rudimentary, usually internal or partly 



concealed by the mantle. 111. Gen. Limax, Parmacella, Testa- 



cella. 

 Fam. 3. Oncidiadcc. Shell wanting. Animal slug-like. 111. Gen. 



Oncidium, Vaginulus. 

 Fam. 4. Limnaidm. Shell thin, horn-coloured, well developed. 



Aperture simple, lip sharp. 111. Gen. Limncea, Physa, Ancylus, 



Planorbis. 

 Fam. 5. Auriculida;. Shell spiral, with a homy epidermis; aperture 



elongated, denticulated. 111. Gen. Auricula, Conovulus. 



Division II. Operculata. Shell ^ivilh an operculum. 

 Fam. 6. Cyclostomida. Shell spiral, rarely elongated, often de- 



pressed. Aperture nearly circular. Operculum spiral. 111. Gen. 



Cyclostoma, Cyclophoriis, Pupiria. 

 Fam. 7. Aciculida. '-'imii elongated, cylindrical ; operculum thiii, 



and sub-spiral. Gen. Acicula, Geometania. 



