MOLLUSCA 



249 



The razor clams, of which Ensis americana is an example, with 

 a greatly elongated shell, are also eaten, both here and in 

 Europe, but they are not so abundant and are difficult to catch. 

 The quahog, or cohog, or thick-shelled clam, Venus mercenaria, 

 is very abundant from Mexico to Cape Cod and forms a com- 

 mon article of food ; other species occur on our Western coast. 

 It was from their shells that the Indians made their wampum. 



All are carnivorous. The largest bivalve 

 of this group is the Tridacna gigas of the 

 Indian and Pacific oceans ; the soft parts 

 of such a clam may weigh ten kilos and 

 the shell as much as two hundred and 

 fifty kilos. 



A boring mussel, the Teredo navalis, or 

 shipworm and allied species, belong here. 

 The Teredo (Figs. 258 and 259) bores 

 tubes for itself in piles, logs lying in the 

 water, and wooden vessels, until the struc- 

 ture crumbles under the slightest strain. 

 Although thousands of these animals may 

 be in the same piece of wood, their tubes 



Teredo navalis. 



natural size, in a piece of tim- 

 ber. P, pallets enlarged ; SS, 

 siphons; J, tube; !\ valves 

 of shell enlarged. (From the 

 Cambridge Natural History.) 



Fir,. 259. Teredo /nivalis, the shipworm, a mussel « hich 

 bores in wood; a fragment of very hard wood rid 

 with the tubes of this mollusk ; the tubes art- lined with 

 a calcareous secretion. ( Phot< graphed tiom specimen 

 by the author.) 



