688 ON MOLLUSCS 
surface a calcareous layer as hard as ivory and of a different colour, 
which covers the preceding layers}. 
Many bivalve and univalve shells have on their inner surface 
lively and glancing colours, which must not be confounded with 
the external colours. They are the play of colours from the refrac- 
tion which is seen to arise on the transmission of white light 
through a prism, and which causes the beautiful phenomenon of the 
rain-bow. The mother of pearl therefore is not to be ascribed to 
any special colouring matter, but to very fine streaks and folds of 
the internal membrane of the shells?. It continues even after the 
carbonate of lime has been removed by an acid, and CARPENTER 
observed that the play of colour disappeared when he extended the 
membrane with needles so that the folds were obliterated’. These 
changes of colour are particularly beautiful in the genus Haliotis, a 
species of which is used by the Japanese to ornament the lackered 
furniture of their houses. 
Whenever the fluid, which forms this innermost layer of bi- 
valves, is effused in the form of small drops, pearls arise, which 
have frequently a very irregular shape. These may accordingly 
exist in very different species of bivalves, also in turbinate uni- 
valves; yet they arise usually in bivalves, and especially in Melea- 
grina margaritifera and Unio margaritiferus. The former species 
of conchifer is fished in the Persian gulph, the last is found in 
fresh water in different parts of HKurope, and also in our country. 
According to Homer, pearls have undeveloped eggs for a nucleus, 
which have remained accidentally on the outside of the mantle 
within the shell. Such may often be the case, but there is no 
1 BRUGUIERE in the Jowrnal d’Hist. Nat. 1. 1792, pp. 307—315, quoted by 
ScuweiccerR Handb. der Naturgesch. der skelettlosen wngeglied. Thiere, s. 681. A dif- 
ferent opinion of BRUGUIERE, that the animal of Cyprea leaves its shell, and forms a 
new one, though adopted by LAMARCK and SCHWEIGGER, can no longer be defended ; 
compare DESHAYES in the new edition of LAMARCK Hist. nat. des Animaua s. Verte- 
bres, X. p. 485. 
* This was first announced by BrewstER Phil. Trans. 1814. If a drop of sealing- 
wax be allowed to fall on a mother of pearl shell, the surface of the wax, which has 
been in contact with the shell, presents the same colours. It may be supposed that 
this was an impression of the surface. But it appears, that in this experiment a thin 
scale of the surface of the shell remains adhering to the wax. 
3 Annals of Nat. Hist. x11. p. 382. 
