278 HALIOTID^. 



at the tables of the rich. It requires a good deal of 

 beating and stewing to make it tender. The inside of 

 the shell displays " all the colours i' the rainbow/' or 

 at least 



" Orange and azure, deepening into gold." 



Perhaps the latter description is not pictorially correct; 

 but it poetically suggests the vivid hues which are so 

 exquisitely blended in the Haliotis. The cause of this 

 iridescence has been explained by Sir David Brewster, 

 Dr. Carpenter, Professor Van der Hoeven, and very lately 

 by Mr. C. Stewart. The hypothesis propounded by the 

 first of these observers was that the peculiar appearance 

 is owing to minute striae or grooves on the surface of 

 the nacre, which alternate with others of animal mem- 

 brane. Mr. Stewart is of opinion that the colour is 

 produced by the nature of the laminae, which decompose 

 the light in consequence of the interference caused by 

 the reflection from the two sides of each film, as may be 

 seen in soap-bubbles and the iridescent surfaces of many 

 natural and artificial productions. He further believes 

 that the nacreous or inner layer is only a modification 

 of the previously formed prismatic layer, each layer 

 being composed of particles or prisms mostly presenting 

 an hexagonal outline. The microscopical structure of 

 the shell has been investigated by Carpenter. He says 

 that " calcified laminae alternate with plates of a brown 

 horny substance, much resembling tortoiseshell in its 

 appearance ; and when the calcareous matter has been 

 dissolved away by dilute acid, these horny plates may 

 be easily detached from each other, the basement mem- 

 brane of the adjoining calcified lamina remaining ad- 

 herent to one side of each of them. In immediate con- 

 tact with the horny plates is a thin layer of large cells 

 of a very peculiar aspect. The nacreous laminae, when 



