286 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
the size of the shell, though it increases iu thickness; hence it is difficult to say how old 
mussels become, but it is reasonable to believe that many live to be from 15 to 25 
years old. 
Floods no doubt cause considerable havoc among the Naiades on the oue hand 
and unusual droughts on the other. In the former, millions are washed from their 
beds and often buried deep under mud, sand, or gravel, and destroyed, or are swept 
out into places where at ordinary times there is no water. River beds are changed, 
and wide areas densely populated with these forms are transformed into dry land. 
A letter received from a correspondent a few days ago told of his being iu Honduras 
and seeing a multitude of buzzards congregated together in the valley of a small 
river. On investigating he found that the stream had changed its course over a long- 
distance during a recent Hood, leaving its old bed dry, and iu it were vast numbers 
of decaying Unios. Where droughts dry up streams that are ordinarily perennial 
myriads of mussels perish. 
In many jiarts of the world, and especially in certain localities of the United 
States, a large amount 
of carbonic acid iu tbe 
water works great dam- 
age to the iiearly mus- 
sels. This is esiiecially 
true in the Southern 
States, while the waters 
of the Upper Mississippi 
y alley are generally 
comparatively free from 
it. Uo doubt the epi- 
dermis is developed in 
part to protect the shell 
from the effects of acid, 
but that upon the beaks, 
being rather thin and 
wearing away or becom- 
Fig. 8. — TJnio ventrioosxis Tiax-nes. Showing rest periods. This .specimen was collected jjjo' brolvCU OU aCCOUUt 
eighth year. being Older there and 
more prominent than elsewhere, gives the acid a chance to begin to erode the shell. 
In many cases it eats down into the heaviest shells until they are seriously injured. 
The animal attempts to counteract this by adding on layers to the inside of its shell, 
but any great erosion always iiroduces disease. I have seen some of the heaviest 
shells of Holston River so eroded that they were shapeless and it was impossible to 
tell what they were, and in Florida many Unios are destroyed by having holes actu- 
ally eaten through to the animal. 
We do not know anything of the enemies of the Naiades when they are first born, 
but it is quite likely that such do exist. Experiments seem to show that fish avoid 
the embryos when aware of their xireseuce, but devour the mussels eagerly after they 
have passed through the encysted state. Raccoons and muskrats prey on them, the 
latter carrying the shells out in great numbers along the banks of lakes and streams. 
Many rare shells that are eagerly sought by the collector are thus brought out of 
deei) water, but they are generally weathered or broken when found. Mr. J. B. Upson, 
