MOLLUSCA 59 



merits from its shell. In some species the shell when polished is 

 wonderfully fine mother-of-pearl, with all sorts of brilliant colors. 



Window-glass shell, Placuna placenta, Fig. 42. This curious bivalve 

 has shells so thin and flat that they may be cut into squares of 3 or 

 4 inches and set in window sash for lighting houses. Their pearly 

 translucency is very pleasing, and they are very widely used in some 

 parts of the tropics, for residences, churches and other buildings. 



"The demand for window shell in other countries for the manufacture 

 of lantern shades, screens, etc., in addition to the local use for windows, 

 has so increased that there is danger of exceeding the supply. In the 

 early part of the year the Bureau of Science undertook the planting of a 

 new bed of these shells. Five thousand live shells were taken in Kawit 



PIG. 42. Window shell, Placuna sp., and square cut from a shell. xM- 



and planted on the mud bottom of the esteros and oyster beds at Malabon. 

 I believe these will thrive, if they are not removed by the local fishermen." 

 From repprt of Director Alvin Cox, Phila. Bu. Sc. 



Camto or helmet shells, genus Cassis. These Gastropod shells owe 

 their value to the fact that they are composed of different colored 

 layers white and black for example so that a figure may be carved 

 out of one color with the other color as a background. The carving of 

 cameos, so important in Italy, has, it is said, degenerated from an art 

 to a trade, so that the fashion has greatly declined. 



Giant clams, genus Tridacna. These are the largest of the lamelli- 

 branchs and may reach a weight of 500 pounds. They are found in 

 the oriental tropics and are said to sometimes drown pearl divers who 

 happen to be caught between their mammoth valves. The valves 



