AECACEA. 319 



teeth, closely interlocking with each other. One genus, 

 Trigonia, is alone referred to this family. The shell of this 

 genus is lined with the most lustrous iridescent pearl, and 

 the exterior is covered with ribs or furrows running longi- 

 tudinally in the only recent species known, but transversely 

 in some fossil specimens. 



The shells vary in size according to their place of growth, 

 and the nutriment that is obtained by their molluscous 

 occupant. It may be, that this species furnished the shell to 

 which Dryden has referred in liis Ode to St. Cecilia^s Day. — 



" When Tubal struck the chortled shell. 



His listening brethren stood around. 

 And, wondering, on their faces fell, 



To worship that celestial sound. 

 Less than a God, they thought, there could not dwell. 

 Within the hollow of that shell, 

 That spoke so sweetly, and so well." 



Eamily IX. Arcacea. 



The waves ride by, with their crests of foam, 

 O'er the liquid plains of their billowy home. 

 Bearing in sun-light their trophies along, 

 To the murmuring surge of the wild waves' song. 



The generic type of this family exliibits a distinct and 



