MOLLUSCA: GASTEROPODA. 267 



both ends. Aperture circular. Foot pointed, with symmetrical 

 side-lobes. Gen. Dentalium* 



Fam. 15. Chitonidce. Shell multivalve, composed of eight, transverse, 

 imbricated plates. Animal with broad creeping font ; 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 



(opisthcn] of the body. 



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

 a shell in most. Sexes united. 



Fam. 1. Tornatellidcs. Shell external, spiral, or convoluted; aper- 

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

 Cinulia. 



Fam. 2. Bullidce. Shell convoluted, thin ; spire sm?.ll or concealed, 

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

 Sulla, Cylichna, Philine. 



Fam. 3. Aplysiadce. 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. Pleurobranchidce. 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, Diphyllidia. 



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

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



Fam. 6. Doridce. 111. Gen. Doris. 



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



Fam. 8. Molidce. 111. Gen. Molis, 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. 1 . Firolidcs. 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. 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 operculum. 

 111. Gen. Atlanta, Bellerophon, Maclurea. 

 SECTION B. PULMONIFERA. Eespiration aerial, by means of a pulmonary 



chamber. 

 DIVISION I. INOPEKCULATA. Shell not provided with an operculum. 



Fam. 1. Helicidce. Shell external, capable of containing the whole 

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



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

 concealed by the mantle. 111. Gen. Limax, Parmacella, Testacella. 



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

 cidium, Vaginulus. 



* Dentalium is placed by Professor Huxley amongst the Pteropoda, from 

 its rudimentary head, the neural flexure of the intestine, the nature of the 

 epipodia, and the characters of the larva. 



