THE MYA ARCTICA. 26*9 



becomes extended over them and the margin, gives them 

 that appearance. 



" * Pearls vary greatly in their transparency. The pink 

 are the most transparent; and in this particular they agree 

 with the internal coat of the shell from which they are 

 formed; for these pearls are only formed on the Pinna?, 

 which internally are pink and semi-transparent, and the 

 black and purple specimens are generally more or less 

 opaque. 



" 4 Their lustre, which is derived from the reflection of 

 the light from their peculiar surface produced by the curious 

 disposition of their fibres, and from their semi-transparency 

 and form, greatly depends on the uniformity of their tex- 

 ture and the colour of the concentric coats of which they 

 are formed. That their lustre does depend on their radiat- 

 ing fibres may be distinctly proved by the inequality of 

 the lustre of the * Colombian pearls,' which are filed out of 

 the thick part near the hinge of the pearl oyster (Avicula 

 margaritifera), so that they are formed, like that shell, of 

 transverse laminae, and they consequently exhibit a plate of 

 lustre on one side which is usually flat, and are surrounded 

 by brilliant concentric zones, which show the places of the 

 other plates, instead of the even, beautiful, soft lustre of 

 the true pearls.' " 



SPECIES II. 



MYA ARCTICA, 



OR 



THE ARCTIC OYSTER. 



The shell of this animal is striated, having the valves with 

 two subspinous ridges, and the hinge without teeth. This 

 oyster is discovered in the Arctic shores, generally amono- 

 a species of Alga? ; it is about the size of a bean, of a palish 

 yellow colour externally, and of a milk-white interiorly.* 



* Encyclopedia Londinensis, vol. xvi. p. 428. 



