158 MANUAL OF NATURAL HISTORY. 



terminal ; hinge usually edentulate, with a, 

 transverse septum. Fluviatile. 



3. FAMILY. Pearl-Oysters (Aviculidse). Mantle 



freely open, margins cirrhated ; foot small, 

 cylindrical, with a byssal groove j shell foli- 

 ated, irregular, pearly within, right valve 

 with a notch for the byssus ; hinge-margin 

 straight ; ligament marginal, simple or inter- 

 rupted. 



4. FAMILY. Pinnas (Pinnidse). Mouth with folia- 



ceous lips ; no separate posterior opening ; 

 anal siphon with a long ligulate valve ; gills 

 foliaceous ; shell wedge-shaped, gaping at 

 ventral margin, pointed at dorsal ; hinge 

 lateral, without teeth ; ligament linear, al- 

 most internal. 



5. FAMILY. Clams (Tridacnidse). Mantle closed, 



except for the branchial and anal orifices, and 

 the aperture for the thick, cylindrical, byssi- 

 ferous foot ; shell regular, transverse, truncate ; 

 hinge with two compressed teeth ; ligament 

 external ; muscular scars united, irregular. 



II. SUB-CLASS. UNIMUSCULAR-BIVALVES 

 (Monomyaria). 



Animal with a single adductor muscle for closing 

 the valves. 



I. OEDER. MICROPODS (Micropoda). 



Mantle-lobes entirely free ; siphons none ; foot 

 rudimentary. 



