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154 ILLINOIS ACADEMY OF SCIENCE 
many places. What was gravel one day, is sand the next, or 
an ox-bow forms and the channel is changed over night. Am- 
blema undulata, Quadrula pustulosa, Fusconaia rubiginosa, F. 
coccinea, Tritogonia tuberculata and Eurynia fasciata were the 
dominant clams. The less common species were: Lampsilis 
ventricosa, L. anodontoides, Strophitus edentulus, Anodonta 
grandis, Anodontoides ferussacianus subcylindraceus, Lasmi- 
gona costata, L. complanata, Alasmidonta calceola and A. 
marginata. 
With the exception of Hurynia fasciata, the dominant clams 
are summer breeding forms, i. e., the sex products mature from 
fall to Spring, and the young are discharged during the Sum- 
mer months. The others are the winter breeding species, the 
embryos of which mature in Spring and are set free in Autumn 
and the following Spring. The summer breeders find plenty of 
water when their young are discharged, also plenty of fish to 
which these young glochidia may become attached. Conditions 
appear, therefore, more favorable to the development of this 
type of clam. On the other hand, the Winter breeders find the 
river very shallow and with fewer fish when their young are 
released. This hypothesis seems valid as an explanation of the 
abundance of certain clams at this particular place. 
At Havana, Illinois, the Illinois River is very broad and deep 
all year round. Fish are always plentiful. Therefore both the 
Summer and Winter breeders are in abundance. It would 
appear also that as the Spring rains swell the tributaries of 
the Illinois River, that fish from it will swim up these tribu- 
taries to spawn and bring with them the encysted glochidia, and 
that these are set free in these tributaries. As the late Summer 
and Winter conditions of the Sangamon River are not favor- 
able to the development of these tiny clams, only a few survive. 
Whether or not these explanations are true, remains to be 
proven by more intensive studies. 
Prof. F. E. Wood (1910, p. 558) gives the following count 
and list of clams that he found at White Heath, all of them in 
one heap and empty,—the results of a muskrat raid: 41 Am- — 
blema undulata, 4 Quadrula pustulosa, 1 Fusconaia coccinea, 
7 Lasmigona sp., 3 Tritogonia tuberculata, 7 Eurynia fasciata, 
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