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, 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 are 

 many of the species which are marked with the same va- 

 rieties of colour. Thus we have the purple and the white 

 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 Rectus is 

 both pink and white — the Cylindricus pink and white — 

 the Orbicularis red and white; the same changes might be 

 noticed in other species. These 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 Jllatus, Ovatus, Peru- 

 vianas, Crassus, and Rectus, were very abundant, and com- 

 monly associated. They are usually imbedded in the soft 



* I have seen some very beautiful pearls taken from the Vewus Mer- 

 cenaria and the Anodontje. 



| 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 margin. 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. I. 7 



