298 REPORT OF NATIONAL MUSEUM, 1887. 



curred, and it seems strange that forms of animal production so insig- 

 nificant should have bad any connection therewith. 



PEARL-BEARING MUSSELS. 



The fresh-water mussels are widely distributed over the surface of the 

 globe ; the rivers and minor water-courses of the northern hemisphere 

 contain a great number of species, and individuals of these species are 

 wonderfully abundant. The freshets which swell the streams and tear 

 away their banks, and make their waters turbid with silt, carry into the 

 soft parts of the mussels particles of sand which irritate the delicate tis- 

 sues. This irritation causes a flow of nacreous lymph, which is deposited 

 on and coats over the rough surface of the disturbing atoms, or it may 

 be that a shriveled egg-case of the mollusk itself becomes similarly 

 lodged and causes a similar annoyance and is coated with nacre in the 

 same way. And so a pearl is commenced, and afterward receives coat- 

 ing upon coating until the accumulated deposits of nacre have reached 

 a thickness that gives the pearl not only size but translucency and iri- 

 descence, and if the color is good and the shape symmetrical, a pearl of 

 value, a precious pearl, is formed. And so in Nature's laboratory an 

 aborted egg or a grain of sand is transformed into a thing of beauty. 



THE INVASION OF BRITAIN BY THE ROMANS (B. C. 55). 



The pearls of Great Britain were famous throughout Europe in the 

 century before Christ. They were obtained from the fresh-water mus- 

 sels (Margaritana margaritifera) of the mountain rivers and streams. 

 Doubtless the extent of the fishery was exaggerated, and the value of 

 the yield in pearls greatly overestimated. History has preserved the 

 tradition that it was this source of wealth that tempted the Eomans to 

 the shores of that country in the year 55 B, C, and ancient writers re- 

 fer to the shield studded with British pearls which Cresar* suspended 

 as an offering in the temple of Venus at Rome.t 



It is highly probable that the invasion of Britain by this famous gen- 

 eral was not for the single purpose of punishing the Britons for the as- 

 sistance they had rendered to the Veuiti of Brittany, with whom Caesar 

 was at war, but with an eye to the pearls, which in his time were far 

 more highly prized than at the present day. 



The invasion of Britain by the Romans, in daring and romance must 

 yield the palm to the enterprise and expeditions of the Spanish con- 

 quistadores centuries later. 



THE CONQUEST OF FLORIDA BY DE SOTO. 



"Isever was the spirit of wild adventure more universally diffused 

 than at the dawn of the sixteenth century. The wondrous discoveries 



* His jouni'-y in Britain was attributed by Suetonius to avarice, which bad been 

 kindled by the report of enormous pearls of line quality to be found in that country. 

 t Simmonds. 



