36 



SHELL GALLERY. 



[Case 20 



B-D.] 



through an aperture at the upper part of the shell. A species 

 found in the Red Sea, T. eloiigata, is eaten by the natives, and the 

 shell employed for the manufacture of lime. Tridacna gigas, the 

 largest known bivalved mollusk, sometimes weighs over 500 lb., 

 that exhibited on the floor of the Gallery being 310 lb. in weight. 

 A large pair bordered with gilt copper are used as benitiers or holy- 

 water vessels in the church of St. Sulpice in Paris. Tridacna are 

 found associated in large numbers in lagoons, among coral-reefs in 

 the Eastern and Pacific Seas. The animals are described as pre- 

 senting a beautiful iridescent glare of blue, violet, and yellow, 

 variegated with fantastic markings, 



The Pholadidce, or Piddocks, are very remarkable shells, of an 

 unusually complicated structure, some having the power of boring 

 into rocks, wood, mud, sand, &c. Their shells are white, adorned 

 with prickly sculpture, and, although thin, are strong. The foot 

 is believed to be the principal excavating instrument, but the 

 shell no doubt is used as a file to enlarge the hole as the 

 creature grows. These animals are brightly phosphorescent ; 

 and certain species are eaten at many places on the shores of 

 the Mediterranean. They appear to be indifferent as regards the 



Fig. 24. 



Picklock, or Borer (Pholas dactylus). (From the British coast.) 



1. Animal in the shell : a, foot ; b, siphons ; c, inhalant orifice ; d, exhalant 



orifice. 



2. Shell : e, accessory valves or plates. 



material they bore into; for the common P hoi as dactylus (fig. 24) 

 of our own shores has been found in slate-rocks, mica-schist, coal- 

 shale, new red sandstone, chalk, marl, peat, and submarine wood. 



