PEARLY FRESH- WATER MUSSELS OF THE UNITED STATES. 
285 
I examined, must have had at least a million eggs in its inner gills, and Br. Isaac Lea 
made a calculation that a single Unio -mu.lUi)lieati(.s, our largest species, and one which 
carries the young in all four of the gills, had no less than 6,000,000. 
There is ample need for all these young, for from the time of their birth until 
they die of old age they are constantly beset by euenues and forces that cause their 
destruction. Even nature herself seems bent on destroying them. Many of them will 
survive after being frozen, and an instance is on record where an Anodonta that was 
frozen solid while gravid hatched out its young all right on being thawed out; yet 
millions of them are undoubtedly destroyed by very severe weather. 1 have seen 
tens of thousands of empty shells in spring in the Potomac, after a very severe winter, 
clinging together by the ligament, and in many cases having shreds of llesh still 
attached to them. These probably did not bury deeply enough or the water might 
have been unusually low during hard freezing. No doubt those that live in shallow 
water suffer most from frost, but on two or three occasions I have seen the Uniotiidcc 
of a region almost exterminated after an uncommonly hard and unfavorable winter. 
Dr. James Lewis, an exceedingly able and careful student of North American 
mollusks, believed that many of our pearly mussels are almost absolutely dormant 
during the winter. He held that in the 
summer they make their growth, adding a 
wide concentric layer to the shell. In winter 
the growth practically ceases, but the man- 
tle still deposits a slight amount of shelly 
material and epidermis; but as the shell 
does not increase in size in winter, these de- 
posits of epidermis form a narrow, dark, 
concentric line or band around its border. 
Others have held to the same idea, which is 
probably a correct one. In many cases these 
dark resting lines are marked as plainly on 
the outside of the shell as are those in a sec- 
tion of a tree, and if the theory of Dr. Lewis is correct, we may thus count the age of 
the clams by the rest periods, as we do that of the forest trees by the annual rings. 
In some cases these marks are not plain or even visible, and this is especially so wdth 
many tropical Naiades, which may continue to grow more or less throughout the year. 
It is possible that those of our northern species which do not show these rest rings or 
only have faint indications of them may be more or less active through the winter. 
It is hard to tell just at what age these mussels begin to breed, because this no 
doubt varies with the species, the amount of food, and favorable or unfavorable 
conditions; but I think it may be stated that it is generally at from three to five years 
from the embrym in those species which carry the ymung in the hinder part of the outer 
gills. Those which have both outer gills filled with young do not begin quite so early, 
and in the forms where all four gills are filled I do not think they often begin to 
produce young until they are seven or eight years old. The number of young pro- 
duced by these yaning mussels is quite small conqiai'ed with those of fully adult or old 
siiecimens. But if these younger mussels, say from five to nine years of age, could be 
always thrown promptly back into the water they could at least furnish one, possibly 
two crops of young, which would go a long way toward keeping up the supply, and 
really prove little or no loss to anyone. After reaching maturity but little is added to 
Fig. 7. — Lampsilisluteohts Lam. Female. Sljell 
produced at i)Osterior base. 
