201 



CLASS IV.— TROPIOPODA. 



Body compressed, covered by the mantle, and enclosed 

 in a bivalve, hinged shell ; head not distinct ; mouth 

 with four flattened labial appendages ; a compressed 

 muscular foot attached to the abdomen. 



Acephala, Cnvier. Mollusca Bivalvia. Tponls, a 

 keel, or Tpoir-q, turning ; Trot's-, foot. 



Animal compressed, higher than broad, with two 

 nearly equal and similar sides, and enclosed in a bivalve 

 hinged shell. Head not distinct. Mouth situated an- 

 teriorly, between four flattened labial palpi ; oesophagus 

 generally short ; stomach pyriform ; intestine convoluted 

 within the liver and ovar}% continued along the back to 

 the vent, of nearly equal width throughout. Liver very 

 large, discharging the bile into the stomach by biliary 

 crypts. Circulatory apparatus, a ventricle and an arterial 

 system, and a venous system with two auricles. Two 

 pairs of laminiform transversely striated branchiae, situ- 

 ated between the mantle and the body. Generative 

 system an ovary enveloped in the visceral mass. Ner- 

 vous system very simple ; no cerebral mass, or head ; 

 nor any organs of sense besides those of taste and touch. 

 A generally compressed muscular foot. Two adductor 

 muscles, sometimes approximated so as to seem single, 

 generally distant, for the valves of the shell. IVIantle 

 very large, thin, laminiform, induplicate, and enclosing 

 the body. Shell of two distinct pieces, or valves, cover- 

 ing the mantle, and having at its upper part an elastic 



