GENERA OF SHELLS. H 



the two valves of the shell ; anterior extremity 

 open. 



T. personata, vacillum. 



TEREDO. Tube testaceous, cylindrical, tor- 

 tuous, open at both extremities, independent 

 of the shell, and covering the animal ; shell 

 bivalve, situated posteriorly outside of the 

 tube. 



The teredines do great mischief to ships, by boring 

 their planks, &C. 



T. navalis, palmulatus. 



PHOLADARIA. 



Shell without a tubular sheath, having accessory pieces which 

 do not belong to the valves, and gapes anteriorly ; liga- 

 ment external. 



PHOLAS. Shell bivalve, equivalve, trans- 

 verse, gaping at each side ; various accessory 

 valves above or below the hinge ; inferior or 

 posterior margin of the valves recurved out- 

 wards. 



The additional valves are always smaller than the 

 true valves, and must be considered as accessory, be- 

 cause their number varies with the species. 



P. dactylus, orientalis, Candida, dactyloides, silicula, cos- 

 tata, crispata, callosa, clavata, oblongata, truncata, cunei- 

 formis, pusillus. 



GASTROCHCENA. Shell bivalve, equivalve, 

 subcuneiform, widely gaping ; anterior open- 

 ing very large, oval, oblique ; the posterior 



