120 The Official Guide to the 
shells of which were formerly used to hold artists’ 
colours—are beautifully tinted in the interior; J. 
margaritifera, the pearl mussel, often five inches 
long, secretes a thick coating of “Mother of 
Pearl”’ on the inside of its valves, as well as some- 
times contains detached pearls of some value, and 
is perhaps the m )st interesting. ‘The Swan Mussel, 
Anodonta cygnea, 1s the largest of the fresh-water 
mussels ; it reaches a length of six or eight inches in > 
suitable localities, thriving best where the water is 
nearly stagnant and food therefore abundant. The 
next and last family of the freshwater Bivalves is that 
of DreEIssENID&, Containing the single genus Drezssena, 
known as the Freshwater or Zebra Mussel, D. oly- 
morpha. ‘The claim of this species to be indigenous 
to Great Britain is by some considered doubtful, its 
first-known habitat being the rivers of Russia and 
the Caspian Sea. This belief is strengthened by its 
having been first noticed in this country in 1824, in 
the Commercial Docks on the Thames, where it 
proved to be abundant; but it was found nine years 
after in widely distant localities. Some of the speci- 
mens in the collection are from the Thames ; others 
possess additional interest from having been found in 
Whittlesea Mere, a freshwater lake long since drained. 
In common with the Sea Mussels it has the habit of 
mooring itself by a “‘ byssus,” an example of which 
will be seen—it is also equally gregarious. 
Class II., GASTEROPODA, is devoted to the 
‘“Univalves,” the first family contains the single 
genus JVeritina, and only one British species, /V. 
fiuviatilis, a small globular-banded shell, with a 
shelly operculum, found in all parts of the kingdom, 
from the Orkneys to Cornwall; then follow two fine 
species of Faludina and two minute Byruinia, and 
two others belonging to the genus Aydrodbia, equally 
