302 MARINE SHELLS 



The crystals are translucent in a great degree, ap- 

 proaching to transparent, and of a bluish- white co- 

 lour. The size of the crystals is large. 1 have seen 

 fragments belonging to crystals, which must have 

 been from four to five inches, belonging to the subva- 

 riety trapezienne elargie. 



The surface of the faces o o are usually dull, of a 

 more opaque milky-white than the remainder of the 

 faces, which have a remarkably fine lustre ; the faces 

 corresponding with those of the primitive rhomboidal 

 prism, as P and M display a fine iridescent colour. 



An account of some of the marine shells of the United 

 States. By Thomas Say. 



[continued from page 276.] 



3. T. *Iris. Shell very thin and fragile, pellucid, 

 compressed, transversely oblong-suboval, iridescent, 

 white, with generally a rosaceous disk and one or 

 two anterior rays, with numerous minute concentric 

 wrinkles, and minute, oblique, acutely impressed, 

 equidistant striae crossing them; strise abbreviated 

 before and not attaining the anterior margin, which is 

 narrowed and subacute ; basal edge rectilinear oppo- 

 site to the beaks. 



Length more than three-tenths of an inch. 



Breadth more than eleven-twentieths of an inch. 



Inhabits the southern shores. 



