296 MANUAL OF ZOOLOGY. 



imbricated plates. Animal with broad creeping foot ; branchiae 

 forming a series of lamellae between the foot and the mantle, 

 round the posterior part of the body. 111. Gen. Chiton, Crypto- 

 chiton. 

 ORDER II. OPISTHOBRANCHIATA. Branchiae placed towards the rear 



(ppistheri) of the body. 



Section a. Tectibranchiata. Branchice covered by the shell or mantle; 

 a shell in most. Sexes united. 



Fam. I. Tornatellidce. Shell external, spiral, or convoluted ; aper- 

 ture long and narrow ; columella plaited. 111. Gen. Tornatella, 

 Cinulia. 



Fam. 2. Bullidce. Shell convoluted, thin ; spire small or concealed, 

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

 Bulla, Cylichna, Philine. 



Fam. 3. Aplysiada. 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. PleurobranchidcB. Shell patelliform, or concealed, rarely 

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

 Gen. Pleurobranchus, Umbrella, Tylodina. 



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



Gen. Phyllidia, Diphyllidia. 



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

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



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



Fam. 7. Tritoniada. 111. Gen. Tritonia, Scyll&a. 



Fam. 8. sEolidce. 111. Gen. sEolis, Glaucus. 



Fam. 9. Phyllirhoidce. Gen. Phyllirhoe. 



Fam. 10. Elysiadce. 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. Firolidce. Body large ; branchiae exposed on the back, or 

 covered by a small hyaline shell ; locomotion by means of a ventral 

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



Fam. 2. Atlantida. 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. 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. Oncidiada. Shell wanting. Animal slug-like. 111. Gen. 

 Oncidium, Vaginulus. 



Fam. 4. Limn&idce. Shell thin, horn-coloured, well developed. 

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

 Planorbis. 



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



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

 DIVISION II. OPERCULATA. Shell with an opercuhun. 



