352 | \FHE OCEAN WORLD. 
. 
according to Mr. Bertram, is found in a variety of the mussel, which 
is characterised by the valves being united by a broad hinge, and 
having a strong fibrous byssus, with which it attaches itself to other 
shells, to rocks, and other solid substances. ‘‘The pearl fisheries of 
Scotland,” he adds, ‘“‘may become a source of wealth to the people 
living on the large rivers, if prudently conducted.” Mr. Unger, a 
dealer in gems in Edinburgh, having discerned the capabilities of the 
Scotch pearl as a gem of value, has established a scale of prices which 
he gives for them, according to their size and quality ; and it is now 
a fact that the beautiful pearls of our Scottish streams are admired 
Fig. 152,—Pectunculus aureflua Fig. 153.—Pectunculus Delessertii 
(Reeve). (Reeve). 
beyond the Orient pearl. Empresses and queens, and royal and noble 
ladies, have made large purchases of these gems; and Mr. Unger 
estimates the sum paid to pearl-finders in the summer of 1864 at 
#10,000. The localities successfully fished have been the classic 
Doon, the Forth, the Tay, the Don, the Spey, the Isla, and most ot 
the Highland rivers of note. Scottish pearls are much whiter in 
colour than Oriental. What tint they have is bluish, while those of 
the East are yellowish. Pink pearls are produced by several exotic 
species of Unio. 
The seventeenth family is that of the Trigoniade, with the genus 
Trigonia, of which so many species occurred in the Jurassic period 
in the strata of Europe, but of which two or three alone are now 
left alive in the seas of Australia. 
The eighteenth family, the Arcade, affords between 200 and 300 
species of the genera of Leda, Nucula, Pectunculus, and Arca. Of 
these we shall only at present instance Pectunculus. 
