CLASSIFICATION I47 



Dentition taenioglossate. 



With creeping habit Strombus. 



With pelagic habit {Heteropoda) Atalanta. 



Dentition rachiglossate : Murex, Buccinum. 



Dentition toxiglossate Conus. 



2. Order. Opisthobranchia. — Ctenidia frequently absent, when 

 present behind the heart ; auricle behind ventricle ; mantle when 

 present not fused by its edges to body-wall; shell frequently 

 absent. 



Mantle present (Tectibranchia). 



Foot with broad flat sole; with creeping habit Bulla. 



Foot with winglike parapodia, pelagic Pteropoda. 



With shell (Thecosomata) Styliola. 



Without shell (Gymnosomata) Clione. 



Mantle not developed (Nudibranchia) Doris. 



3. Order Pulmonata. — Ctenidia wanting; mantle fused by its 

 edges to body-wall ; terristrial or aquatic. 



Eyes at base of tentacles (Basommatophora) Pond Snails. 



Eyes at tip of tentacles Land Snails and Slugs. 



III. Class ScAPHOPODA. — Visceral hump developed ; bilaterally sym- 

 metrical; shell cylindrical, open at both ends Dentalium. 



IV. Class Lamellibranchia or Pelecypoda. — Visceral hump not 

 developed ; bilaterally symmetrical ; mantle forms two lateral 

 folds; shell bivalved; anus terminal. 



I. Order Pkotobranchia. — Gill a true ctenidium; pleural gan- 

 glia not united to cerebral Nucula. 



2.- Order Filibranchia. — Gill filaments elongated and bent up- 

 wards at ends; cerebral and pleural ganglia fused Mytilus. 



3. Order Pseudolamellibranchia. — Gill filaments turned up at 

 ends and with interlamellar junctions; cerebral and pleural 

 ganglia united Oyster. 



4. Order Eulamellibranchia. — Gill filaments united to form a 

 platelike gill ; cerebral and pleural ganglia united. 



Venus (Quahog). 



5. Order Septibranchia. — Gill reduced to a muscular perforated 

 septum between the mantle and suprabranchial chambers. 



Cuspidaria. 



