518 LECTURE XXI. 



dinally, and also transversely, into compartments by irregularly 

 placed, incomjDlete, concave septa. 



The valuable pearls of commerce are a more compact and finer 

 kind of nacre, often developed in the substance of the mantle, or 

 around a particle of sand or other foreign body which has gained 

 admission to the pallial cavity. The Meleagrina or Avicula mar- 

 (jaritifera of the Indian seas is most famous for these productions. 

 Those developed in the gills or inner layer of the mantle are small 

 and numerous ; those of the outer layer are the largest, but least 

 regular, and attached to the shell. 



The " external " pearls consist of concentric layers of minute 

 vertical prisms, the " internal" pearls of concentric layers of wavy 

 calcified membrane. The iridescent nacreous lining of the pearl- 

 oysters {Avicula), and many other shells, consists of the same wavy 

 lamelliform tissue : the pearly lustre is due to the diffiaction of the 

 rays of light by the out-cropping edges of the laminae, and, in some 

 cases, to the minute plication of a single lamina. 



If the shell of a living pearl-oyster be perforated, and a minute 

 particle of sand introduced, it becomes a nucleus round which a pearl 

 is developed. Linnceus was knighted on making known to his 

 sovereign this practical application of his science ; but the artificial 

 production of pearls had long been known to the Chinese, who 

 obtain them of definite forms by introducing substances of the re- 

 quired shape into the shell. 



The Unio margaritifera, or pearl-mussel of British lakes and rivers, 

 is fished up for the ornamental excretions to which it is subject. It is 

 probable that the pearls from this source, collected by the ancient 

 Britons, may have given rise to the statement by Tacitus in his Life 

 of Agricola, of " pearls not very orient, but pale and wan," being 

 among the indigenous products of the conquered island.* 



The peculiar shape and development of the foot in the Solen and 

 other " burrowing " bivalves might have led to its recognition as the 

 excavating agent, if even it had not been seen to effect the purpose in 

 the living mollusk. Direct observation of the "boring " bivalves in 

 the act of perforation has been rarely enjoyed, and the instruments 

 have been guessed at or judged of from the structure of the animal. 



The peculiar shape, great strength, and restricted size of the con- 

 centrically ridged valves in the Teredo navalis, the disproportionate 

 size and strength and the red colour of their adductor muscle, with 

 the curved umbonal processes for its advantageous leverage, could 

 not fail to attract the attention of the unbiassed observer to their 



* CCCXIX. p. 164. 



