374 INTRODUCTION TO CONCHOLOGY. 



dead shells^, in which case it forms a complete testaceous 

 covering, entirely foreign to its place of attachment. The 

 shell is inequivalve, inequilateral;, gaping widely on the 

 anterior side, and resembHng an oblong oval ; united by a 

 ligament behind, and having in the interior a small spoon- 

 shaped curvature, a character which becomes more fully 

 developed in shells of the Teredo and Phorus, 



Perforations in the timbers of many gallant vessels are 

 caused by the Teredo navalis, or common Ship-worm, of 

 which the generic appellation is derived from a Greek word 

 signifying to bore. The formation of this curious mol- 

 lusk bears an obvious reference as well to the purpose for 

 which he is designed, as to the substance in which he 

 becomes enclosed. The head is well prepared for en- 

 countering difficulties, being surmounted by a helmet, and 

 pro\'ided with a tooth adapted for perforating the hardest 

 substances ; the neck is also furnished with strong muscles, 

 which materially assist the operations of the head, and the 

 body is covered with a kind of transparent horn. The 

 heart is said to be situated in the back, and admits of only 

 a single circulation, a peculiarity obvious in many animals 

 that breathe in water. In like manner the extraordinary 

 fact, that the breathing apparatus of the ship-worm, in 



