MOLLUSCA : GASTEROPODA. 341 



Fain. 2. Bullida. Shell convoluted, thin ; spire small or concealed, 

 lip sharp. Animal more or less investing the shell. 111. Gen. 

 Bulla, Cylichna, Philine. 



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

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

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

 Dolabella. 



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

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

 Gen. Pleurobranchus, Umbrella, Tylodina. 



Fam. 5. Phyllidiadce. Animal shell-less, covered by a mantle. 111. 



Gen. Phyllidia, Dlphyllidia. 



Section b. Nudibranchiata. Animal destitute of a shell in the 

 adult condition. Branchice external, on the back or sides 

 of the body. 



Fam. 6. Doridee. 111. Gen. Doris. 



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



Fam. 8. Ai.olidce. 111. Gen. SEolis, Glaucus. 



Fam. 9. Phyllirhoidce. Gen. Phyllirhoe. 



Fam. 10. Elysiad<z. 111. Gen. Elysia, Acteonia. 

 ORDER 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. III. 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 



pulmonary chamber. 

 DIVISION I. INOPERCULATA. Shell not provided with an 



operculum. 

 Fam. i. Heliddff. Shell external, capable of containing the whole 



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

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



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



cella. 

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



Oncidium, Vagimilus. 

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



Aperture simple, lip sharp. 111. Gen. Limn&a, Physa, Ancylits, 



Planorbis. 

 Fam. 5. Auriculidce. Shell spiral, with a horny epidermis ; aperture 



elongated, denticulated. 111. Gen. Auricula, Concvulus. 

 DIVISION II. OPERCULATA. Shell with an operculnm. 

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

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



Cydostoma, Cydophorus, Pupina. 

 Fam. 7. Adculida. Shell elongated, cylindrical ; operculum thin 



and sub-spiral. Gen. Acicula, Geomelania. 



