Remarks on the Unios of the United States. 45 
Very few regular pearls have been found in our Unios.* 
Calcarious deposits are frequently made on the nacre around 
any foreign matter, such as gravel, sticks, and mud, which 
the animal cannot easily eject. The U. alatus is peculiarly 
liable to this kind of deposit on its nacre. It may be here 
remarked J that while the periostracha or epidermis appears 
to be secreted by the mantle of the animal, these pearly de- 
positions seem to arise from a fluid exuded from the body. 
It is a curious subject of speculation what circumstances 
occasion the splendid colours which often embellish the in- 
terior surface or nacre of the Unios. These colours are per- 
haps never characteristic of any species; at least there arc 
many of the species which are marked with the same va- 
rieties of colour. Thus we have the purple and the wdiite 
Matus, though this last variety rarely occurs — we have the 
purple and the white Fluviatilis^ — every shade of pink and 
orange is also common in this last species. The Beclus is 
both pink and white — the Cylindricus pink and white — 
the Orbicularis red and white; the same changes might be 
noticed in other species. Tliese different colours cannot de- 
pend on the food of the animal, as they are often found in 
the same species in the same locality.! 
Having carefully examined the rivers in the neighbour- 
hood of Pittsburg, I found that the Alatus^ Ovatus, Peru- 
vianus, Crassus, and Beetles, were very abundant, and com- 
monly associated. They are usually imbedded in the soft 
* I have seen some very beautiful pearls taken from the Venus Mer« 
cenaria and the A?foi)oiirTiE. 
•j- The difference of colour noticed above may be seen in some marine 
bivalves, particularly the Venus Mercenaria, or common clam — some of 
which are perfectly white, while others are of a deep brilliant blue, 
round the inner marg-in. The whole surface, it has been said, is some- 
times blue; but this must be an exceedingly rare occurrence, as many 
thousands have been examined without success. 
Vol. L 7 
