TEREDO. 125 



of the water according to necessity. Tliere appears 

 however to be no more reason for classing these 

 shells with the multivalves, than Turbines and 

 others which possess an operculum with the bi- 

 valves. The only difference between the lid of the 

 Teredo and that of a Turbo is^ that the one is con- 

 structed of four pieces^ the other of a single disk. 

 These pieces cannot constitute the shelly because 

 the animal cannot be said to inhalit them, as it 

 does the testaceous tube. Neither is there any 

 analogy between them and the accessory valves of 

 Pholas. 



At first sight the Teredo may easily be mistaken 

 for a Serpula, and in many cabinet specimens the 

 valves are lost. There are, however, but three spe- 

 cies of the former genus at present known, and their 

 external characters are soon to be distinguished. 

 The first, T. navalis, which is the most common, 

 is much more thin and brittle than Serpulse in ge- 

 neral, especially towards the smaller end. The 

 name Teredo has probably been attached to these 

 shells on account of their round smooth figure. 



