30 SOME AUTUMN DAYS IN IOWA 
be a bit of coral which the marble men call birds- 
eye, but which scientists call Acervularia David- 
sonii. The long name means that the piece of 
coral is shaped like a cake and was discovered by 
Davidson and so the piece of coral might be called 
Davidson’s coral-cake. It is a most beautiful 
structure. The upper surface shows a fine pattern 
of six-sided cells, about a .quarter inch in diameter. 
A delicately crenulated wall runs around each cell. 
In the middle of the cell there is an apparent small 
cylindrical wall. From this radial lines run to the 
outer wall — these are called septa. Now if you 
bring a piece of this coral home and take it to a 
marble cutter he will split the cake vertically for 
you and polish up one side. That process will 
divide the space between each two septa into deli¬ 
cate horizontal dissepiments and the polished side 
of the coral will look like a piece of beautifully 
woven cloth. Then when you go to the Field 
Museum in Chicago you may visit the geology cases 
and find a specimen just like yours. You may find 
the ledge here mentioned at Midriver Park. Far¬ 
ther back on the bank you may pry some fine speci¬ 
mens out of the clay, which is said to be their 
natural home. If you have a bright member of 
the high school geology class with you he will name 
many other interesting specimens which you find 
in the same place: Favosites Emmonsi, Helio- 
phyllum halli, and several others. 
