16 THE NAUTILUS. 



Having thus outlined the physical conditions of their environment, 

 it may perhaps be well to s{)eak of the manner in which the 

 Unionidoe migrate from one locality to another. 



The number of young produced by the species of this family is 

 simply enormous. Lea counted some 600,000 perfectly formed 

 embryos in the ovaries of an Anodonta undulata, and estimated that 

 a female Unio multiplicahis contained no less than 3,000,000 shells. 



It is believed that these young bivalves sometimes attach them- 

 selves to fishes and are thus carried from place to place ; no doubt 

 others are taken up in mud on the feet or feathers of aquatic birds, 

 and may survive a short transit through the air, and certainly 

 they might be swept from stream to stream across the country dur- 

 ing the time of floods, when almost the entire surface of that level 

 area is covered with water, in many places flowing with a rapid 

 current. During the rainy seasons in South Florida I have repeat- 

 edly seen the whole country a sheet of water, with myriads of fish 

 swimming in every direction among the palmettoes and over the 

 fields. That the Unionidce are carried out over the land is, I think, 

 proved by the fact that I have found Unio obesus existing in great 

 numbers in low places and drains in the piney woods of South 

 Florida, at quite a distance from any stream, where there was not 

 a drop of water outside of perhaps three months of the rainy season, 

 and where during the remaining nine months of the ^year they 

 must have lain dormant in slightly damp sand. I have dug these 

 unios alive out of such sand banks in such places, and during the 

 dry season, by the bushel. 



It is a well-known fact and one which seems to me much more 

 strange than the migration of unios across such a country, that 

 artificial ponds and reservoirs often become densely peopled with the 

 Naiades, even when their outlets are altogether too insignificant in 

 size to be a residence for these molluscs. In such cases it would 

 seem most probable that aquatic birds had been the means of such 

 distribution, and possibly in rare instances they may have been lifted 

 from their original homes and carried by cyclones. 



But once having reached the lower part of the St. Johns River, I 

 cannot conceive of any difficulty mollusks would encounter in 

 spreading toward its sources. The stream is really little more than 

 a freshwater estuary for a long distance from its mouth, and has but 

 a few feet of fall throughout its entire length. Let any of the 

 Unionidce be placed in any part of a stream and if the conditions 



