230 



ZOOLOGY 



becomes replaced by the two found in the adult ; the gills 

 arise, the shell gains its adult character, 

 and becomes free from the fish on which 

 it has hved temporarily as a parasite. 



There are other species of fresh-water 

 lamellibranchs besides the Unionidae. 

 Thej' have much the shape of our com- 

 mon hard-shell clam of the sea, but are 

 much smaller in size, rarely exceeding 

 fifteen millimetres. Cy clas is of world- 

 wide distribution, but it is especiallj^ 

 common in North America. It occurs 

 in ditches, ponds, and rivers of New 

 England, as well as in the South and 

 West. Some species of the genus in- 

 habit brackish water. 



The remaining families which we 

 shall consider come from the sea, and all 

 occur on our Eastern coast. Three 

 grand divisions may be recognized : (1) 

 the burrowers (Pholadidse, Teredidse, 

 Solenidff, Mj-idae) ; (2) the heavy- 

 shelled bottom feeders (Mactridse, 

 Veneridff, and Arcidse) ; (3) the byssus 

 spinners (Mytihda-, A\'icuhda?, Pec- 

 tinidae, Ledidse, and Ostreidte). 



The Teredida;! and Pholadida; = in- 

 clude certain wood and rock boring 

 species, which do great damage by bor- 



FiG. 21?,. — Teredo naval 

 in a pieor of timl)er. 



P. 



pallets; SS, siphons; T, 

 tubo ; V, valves of shell. 

 From the "Cambridge 

 Natural History." 



' lereo, to bore. 



^ pholas, Greek name for a boring mussel. 



